Okay, I realize this may be a foolish question, but regarding effects that say "once per turn", can they happen multiple times in a round or are they like reactions where you get one per each of your own turns? For example, could the Savage Attacker feat trigger on both your turn and on an opportunity attack on someone else's turn? If this is the case, a rogue's sneak attack is also "once a turn", surely that isn't allowed on such things?

Once per turn means once on each creatures turn so it can happen multiple times per round. In reality it is a maximum of twice per round for such features if they require a reaction (and I can't think of any that don't.

I'm not going to do through the maths but rogus need some round with an extra sneak attack to keep up with other classes.

If all it says is "once per turn" then it can theoretically happen as many times in a round as there are turns. Sneak attack is actually the prime example; you can absolutely sneak attack on an attack of opportunity if the other circumstances line up, potentially getting two sneak attacks in a round.

Just don’t get it confused with “once on your turn” which means it’s only your turn so only once per round. 

I don't know how to reply to all three of you without separate messages so I'm just hoping you all see this, thanks for confirming my thoughts! I was fairly certain that was the case but wasn't entirely.

I'll just give one example to add to the information here:

Warlock - Eldritch Invocations - Eldritch Smite:
"Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller."

So if the warlock has multiple attacks, it will only be able to apply this to one of its attacks in a single turn.  BUT if it is able to attack, and hit, on another creature's turn, it may invoke it again.

This is how the "Once Per Turn" note generally works.  It's a restriction to something that can already occur on any creature's turn (hit a creature) but prevents it from occurring more than once in a turn.

Other examples are the Monk - Way of Mercy "Hand of Harm" ability restricted to inflicting extra damage only once per turn. Monk - Way of the Drunk Master "Mind of Mercury" which allows you to spend a ki point to take another reaction. Wizard - School of Divination "Portent" replace rolls of you or another creature but only once per turn.

As info complement, here's an official ruling on the question of ''once per turn'' in the Sage Advice Compendium actually;

Can a rogue use Sneak Attack more than once per round? The Sneak Attack description specifies that you can use the feature once per turn, but it’s not limited to your turn. The feature also doesn’t limit the number of times you can use it in a round. You sometimes get a chance to use Sneak Attack on someone else’s turn. The most common way for this to happen is when a foe provokes an opportunity attack from you. If the requirements for Sneak Attack are met, your opportunity attack can benefit from that feature. Similarly, a fighter could use Commander’s Strike to grant you an attack on the fighter’s turn, and if the attack qualifies, it can use Sneak Attack. Both of those options rely on the use of your reaction, so you could do only one of them in a round. Because you get only one reaction per round, you’re unlikely to use Sneak Attack more than twice in a round: once with your action and once with your reaction

Voice of Authority

Starting at 1st level, you can invoke the power of law to embolden an ally to attack. If you cast a spell with a spell slot of 1st level or higher and target an ally with the spell, that ally can use their reaction immediately after the spell to make one weapon attack against a creature of your choice that you can see.

If the spell targets more than one ally, you choose the ally who can make the attack.

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Silvery Barbs

Casting Time: 1 reaction, which you take when a creature you can see within 60 feet of yourself succeeds on an attack roll, an ability check, or a saving throw
Range: 60 feet
Components: V
Duration: Instantaneous

You magically distract the triggering creature and turn its momentary uncertainty into encouragement for another creature. The triggering creature must reroll the d20 and use the lower roll.

You can then choose a different creature you can see within range (you can choose yourself). The chosen creature has advantage on the next attack roll, ability check, or saving throw it makes within 1 minute. A creature can be empowered by only one use of this spell at a time.

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Order Clerics who get Silvery Barbs (via dipping or feats) can allow a Rogue to use their reaction to attack with Advantage when it's not the Rogue's turn, so it's entirely possible for a Rogue to get a Sneak Attack on their turn and then later in the round get another Sneak Attack from this combo, or similar effects.