When using a Pure Martial Class, not counting spells from Feats, Racials or Items. Even with more than one Martial in a Multiclass, you can make as many reactions as your Proficiency Bonus. Each reaction

Example as a Barbarian with +2 Proficiency bonus, if multiple enemies surrounding you move away, granting opportunity attacks, you may attempt to attack only one target that opened an opportunity attack as they move away as you would only have one Attack.

A Monk of the same Level could use both attacks if they used their second Proficiency Bonus reaction with their unarmed Strike ability. Or if not attacking they could catch multiple Projectiles up to their Proficiency Bonus.

As a Half Caster You Gain half the proficiency Rounded down if you are using a weapon with the Loading, Heavy, Reach or Special properties Even with the Warcaster Feat. Half your Proficiency Bonus Rounded up if using Finesse, Light or Thrown weapons.

Example as a Ranger with +3 Proficiency Bonus, wielding Hand axes. If you chose to hold your action during your turn, with the condition you will attack if any targets move towards allies. You are granted normal class multiattack rules as normal, so using 2 attacks using a ranged weapon or Moving as one reaction and attacking with a melee weapon once as the other reaction.

Full Casters retain their Single reaction, unless Multiclassed with a Full Martial, then they would be rounded up or down based on the higher class level, rounded down if even.

 I am picturing a 20th level Echo Knight with 6 reactions every turn.... and the ability to surround an enemy with themself and 2 echos, and if the thing moves get 3 AoOs. 

I am now picturing that same scenario with an 18 level echo knight with 2 levels of paladin, doing the same thing, but smiting on 2 of the 3 AoOs as well.

Can you elaborate on why you think the martials are at a disadvantage where they need 1-5 more reactions?

All this would really do is give a minor boost to martial, while MASSIVELY boosting casters that take a level dip. Now instead of just getting some armor proficiency, they also got extra reactions to cast shield, counter-spell, or temporal shunt, greatly increasing their defensive capability. Meanwhile, most martials don't get too many reaction abilities (Rouge has some good ones, but I don't see them on the list). Martial classes do need a significant buff, but this is not good enough on its own as a buff, and requires some level of optimization to use effectively (which, in my opinion, is a bad thing; some optimization should be possible, but not needed to make an ability good.) It's also not specific enough, and, as stated above, could give a good buff to non-martials, so it wouldn't help to widen the power gap between the two groups.

I would probably just say "If you have extra attacks, you also have extra reactions that can only be used for opportunity attacks".

Casters get no implied bonus, Casting limitations would still apply, one spell per round and one cantrip with a reaction. So small dips into Martial, would still fall short of Half casters until their Martial class level was higher than Full caster class level. Otherwise there will be no benefit in terms of less than half levels in Martial, at which point it is a sacrifice on Full casting features.

Full Martial = +Full PB

Half Caster = 11 Martial + 9 Full Caster

If Full Caster Level is > Martial Level, No PB based reactions are added.

If that is clearer. Thanks for highlighting its clearer with that defined.

Quote from MARSHalMELLOW >>

Casters get no implied bonus, Casting limitations would still apply, one spell per round and one cantrip with a reaction.

 Where are you getting the rule on casting limitation that you can only cast one spell per round and that the spell cast as a reaction must be a cantrip?

RAW, you can normally cast up to two spells on your turn, one as an Action and one as a Bonus Action (although if you cast a BA spell, the only spell you can cast using your Action is a cantrip). You can cast one reaction spell per round, but thats not limited by your casting thats limited by the fact that you usually get only one reaction per round. This spell can be a leveled spell or possibly a cantrip if using War Caster. Either way, if you give a Caster more reactions, they can cast more reaction spells. 

I honestly don't know what you're attempting to accomplish here. Most rounds, the martials or half-casters wouldn't be able to use their reactions without an ability that lets them do so use a reaction without responding to something. Abilities like that would become incredibly broken, - which would make some half-caster builds way more powerful - and everything else would be pretty much the same, though a Warrior might get to make on extra opportunity attack every couple of combats.

The fact that I can create an Arcane Trickster with this build and cast Silvery Barbs twice or Shield twice in the same round of a super big fight at level 10 is very much broken, and spells such as Hellish Rebuke would be worth a lot more from Tieflings thanks to the possible extra uses of them.

All in all, this is a cool idea but it wouldn't really do much to help the Warriors. It would just give min-maxers ways to make some super powerful builds.

Quote from MARSHalMELLOW >>

 

If Full Caster Level is > Martial Level, No PB based reactions are added.

 That would fix the issue, but seems like a weird distinction to make that is not used anywhere else in this game. But it's still flawed. The extra reactions would still be broken for half-casters like Paladins or Arcane Trickster that get their hands on good reaction spells, and it would make some Subclasses broken while barely benefiting others. By default, Fighters don't get any good reaction abilities; sure, it would let them make extra opportunity attacks, but that's not something you can intentionally trigger in most cases, so isn't a significant damage buff. However, it would make some builds really powerful. It is a very convoluted buff that is not only weirdly complicated, but also isn't significant enough on its own to be meaningful.

Quote from KthunkTheBarbarian >>

 

Quote from MARSHalMELLOW >>

 

If Full Caster Level is > Martial Level, No PB based reactions are added.

 That would fix the issue, but seems like a weird distinction to make that is not used anywhere else in this game. But it's still flawed. The extra reactions would still be broken for half-casters like Paladins or Arcane Trickster that get their hands on good reaction spells, and it would make some Subclasses broken while barely benefiting others. By default, Fighters don't get any good reaction abilities; sure, it would let them make extra opportunity attacks, but that's not something you can intentionally trigger in most cases, so isn't a significant damage buff. However, it would make some builds really powerful. It is a very convoluted buff that is not only weirdly complicated, but also isn't significant enough on its own to be meaningful.

 Any full martial with enough dip to rogue could uncanny Dodge out the wazoo with that….

Some examples I had in mind were:

1. Characters with the Feat Mage Slayer, if they were surrounded by spell casters, could use more reactions to defend themselves from spells.
2. Battlemaster Fighters would be really effective, but consume superiority die resources quickly.
3. Monks catching projectiles, in the face of multi attacks. 
4. Arcane tricksters do have access to many good reaction abilities, but generally will consume more resources in terms of spell slots. As half casters, it really fits with their theme too.
5. The Echo knight I would classify as a half caster, due to their ability to summon echoes.
6. There are limited positions additional reactions would be advantageous, not including casting Shield 2 times and they would not stack, the longest duration effect would prevail.
   
   If it meant taking a risk to take advantage of multiple reactions as a mage slayer or a Monk, and players are making decisions that will add suspense, it is a good thing.

this could be an appropriate balance too, Many classes getting a Multi attack, then Monks getting their bonus attack and can do an extra with flurry of blows.

arcane tricksters would not get their second attack or uncanny dodges, unless they dual wield, getting a second offhand attack earlier, that could be used to uncanny dodge once more at the cost of a free hand for somatic spell components.

I would class an echo knight, magically summoning echoes and all, as a rather unique half caster.

If the players position themselves well enough to commit multiple smites they should feel rewarded and worried if they wasted them on illusions, they are using resources on smites after all.

Can you elaborate on why you think the martials are at a disadvantage where they need 1-5 more reactions?

I generally saw Martials defending casters, with one hand behind their backs. More than one enemy surrounding them to move past, with nothing they can do. 

I may have over short handed the RAW, Sorry if anyone misunderstood:

1. Action Spell.
2. Bonus Action (although if you cast a BA spell, the only spell you can cast using your Action is a cantrip).
3. with a Reaction (spell or any other class/ feat reaction).

With many reaction spells not being useful to use more than once per round, like Shield. (https://www.dndbeyond.com/sources/phb/spellcasting#CombiningMagicalEffects)

Any that are would still be using resources. Having full health and no shield slots before a bigger fight isn't the best position to be.

If you wanted to avoid caster abuse, I'd suggest making it specific to opportunity attacks - or maybe classify things as 'opportunity actions' and add wording to features like Uncanny Dodge to identify them as opportunity actions.

But whenever the martial/caster divide comes up, I'm gonna say - it's not a combat problem, it's an out-of-combat problem. You can make fighters gods of the battlefield and they'll still be sitting there picking their nose for half the session at 18th level while the casters are circumventing challenges left and right with spells and magical features.

The simplest solution for this concept would simply be : "When you use your reaction to make a single weapon attack, you instantly regain the use of your reaction after making that attack."