In each round, you can take one action, one bonus action (if a feature grants it to you), and one reaction. That applies to monsters too, although legendary monsters also get legendary and maybe lair actions as well.
Many monsters have multiattack which allows thm to use multiple abilities in a single action. The description will say exactly what they can do, for example, an Adult Black Dragon it says
Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
Therefore it can do the multi attack as a single action it can also do any of the other actions (e.g. Acid breath) but then can not use any other abilities.
https://www.dndbeyond.com/monsters/2560807-evoker-wizard
If we take the Evoker wizard as an example of a monster in monsters of the multiverse we have the following actions that can be taken.
3 arcane bursts.
Sculptured Explosion
Or a list of spells.
Am I right in thinking that I can only do 1 of these things a round?
In each round, you can take one action, one bonus action (if a feature grants it to you), and one reaction. That applies to monsters too, although legendary monsters also get legendary and maybe lair actions as well.
DMing:
Dragons of Stormwreck Isle
Playing:
None sadly.
Optimization Guides:
Literally Too Angry to Die - A Guide to Optimizing a Barbarian
What they said☝️. Monsters have the same action economy as PCs.
Many monsters have multiattack which allows thm to use multiple abilities in a single action. The description will say exactly what they can do, for example, an Adult Black Dragon it says
Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
Therefore it can do the multi attack as a single action it can also do any of the other actions (e.g. Acid breath) but then can not use any other abilities.
Monsters and NPCs, like player characters, have ;1 action/turn, 1 bonus action/turn, 1 reaction/round and unlimited no action required elements.