Hey fellow DMs, I wanna pick your brains for a moment. I've been finding myself with a plethora of monsters on the field, and taking their individual turns can be rather harrowing.

So I was wondering, rather than going with the Minion Variant Rule, what are your opinions on making these large groups of enemies into "Swarms?"

Mainly in the case of Curse of Strahd (Which I'm running right now), my party is in the Wizard of Wines Winery. It calls for 24 Twig Blights to be hiding inside one of the fermentation vats. That's... A lot of blights. I was thinking of turning them into maybe two swarms. Not only that, but it also calls for 30 Needle Blights in six groups of five. Five of them is manageable, but assuming they all get pulled together, do you think it would be beneficial to run them in swarms?
I would end up buffing their HP, and having two sets of attacks - One at full health that hits hard, but not too hard. And one for half health and lower calling for maybe half the amount of dice.

I was thinking for health, 2/3 the total health of the grouping so it isn't obnoxious, with a 2d6 on the attack for the Twig Blight Swarm, 1d6 when cut down to half?

Just thought I'd pick your minds and see. It's mainly so I don't have to bog down combat with.... 24 Twig Blights and/or 30 Needle Blights depending on how FUBAR the encounter can get.

I have to assume they're intending you to run the blights like swarms, because the idea of rolling initiative for 30 separate creatures (plus reinforcements) sounds nuts. I think the ideas you have here are basically good; the only other suggestion I would offer is to have the swarms take additional (maybe double?) damage from AoE spells and effects that cover the entire swarm. I assume the frankly absurd idea of 24 Twig Blights in a cask is intended to be balanced by the fact you could take out almost all of them with a single, well-placed spell. By grouping them into swarms that have more HP and only get hit once by area effects, you accidentally make the groups significantly more durable. AoE vulnerability could help counteract that.

You could always give the blights the Swarm trait but you would have to homebrew their statblocks accordingly.

FYI there's also guidelines in the Dungeon Master Guide for Handling Mobs

Handling Mobs

Keeping combat moving along at a brisk pace can be difficult when there are dozens of monsters involved in a battle. When handling a crowded battlefield, you can speed up play by forgoing attack rolls in favor of approximating the average number of hits a large group of monsters can inflict on a target.

Instead of rolling an attack roll, determine the minimum d20 roll a creature needs in order to hit a target by subtracting its attack bonus from the target’s AC. You’ll need to refer to the result throughout the battle, so it’s best to write it down.

Look up the minimum d20 roll needed on the Mob Attacks table. The table shows you how many creatures that need that die roll or higher must attack a target in order for one of them to hit. If that many creatures attack the target, their combined efforts result in one of them hitting the target.

For example, eight orcs surround a fighter. The orcs’ attack bonus is +5, and the fighter’s AC is 19. The orcs need a 14 or higher to hit the fighter. According to the table, for every three orcs that attack the fighter, one of them hits. There are enough orcs for two groups of three. The remaining two orcs fail to hit the fighter.

If the attacking creatures deal different amounts of damage, assume that the creature that deals the most damage is the one that hits. If the creature that hits has multiple attacks with the same attack bonus, assume that it hits once with each of those attacks. If a creature’s attacks have different attack bonuses, resolve each attack separately.

This attack resolution system ignores critical hits in favor of reducing the number of die rolls. As the number of combatants dwindles, switch back to using individual die rolls to avoid situations where one side can’t possibly hit the other.

Mob Attacks

d20 Roll Needed	Attackers Needed for One to Hit
1–5	1
6–12	2
13–14	3
15–16	4
17–18	5
19	10
20	20