I just want to make sure I understand the 1 reaction per turn rule correctly. As an opportunity attack is a reaction (PHB p.195), if my fighter has already used a protection fighting style, if an NPC disengages, he cannot make an attack. Is that correct?
I assume this is unfortunate wording from your part because if a creature disengages (as in, uses disengage action) you don't get attack of opportunity anyway (unless you have a way of ignoring that, like Sentinel Feat) - so I will assume you meant simply "leaves your threat area".
Yes, if you used your reaction for something else, you don't get another one until the start of your next turn.
I assume this is unfortunate wording from your part because if a creature disengages (as in, uses disengage action) you don't get attack of opportunity anyway (unless you have a way of ignoring that, like Sentinel Feat) - so I will assume you meant simply "leaves your threat area".
Yes, if you used your reaction for something else, you don't get another one until the start of your next turn.
Unless you are specifically Cavalier Fighter 18. They are the only exception I'm aware of RAW, and they still only get one reaction per each character's turn. In the scenario described, if a creature attacked someone, the Cavalier used their reaction for protection style, and then the attacking creature used its remaining movement to trigger an AoO, the Cavalier would be unable to make the AoO since they'd used a reaction on that turn. If the attacking creatures buddy moves the next turn (not round), the Cavalier can make any AoO that is triggered, but wouldn't be able to do protection style or cast Shield or anything else that used a reaction that wasn't an AoO (possibly even Warcaster spell casts, depending on your DMs interpretation, since the spell cast isn't an AoO even though it's replacing an AoO. The scenario is so niche that I wouldn't expect much of a clarification on it RAW, but I think think the RAI is clear enough that most DMs would allow it.)
However, anyone that isn't at least Cavalier 18 won't be able to use a reaction again until after their turn as Lathlaer said.
I just want to make sure I understand the 1 reaction per turn rule correctly. As an opportunity attack is a reaction (PHB p.195), if my fighter has already used a protection fighting style, if an NPC disengages, he cannot make an attack. Is that correct?
I assume this is unfortunate wording from your part because if a creature disengages (as in, uses disengage action) you don't get attack of opportunity anyway (unless you have a way of ignoring that, like Sentinel Feat) - so I will assume you meant simply "leaves your threat area".
Yes, if you used your reaction for something else, you don't get another one until the start of your next turn.
Unless you are specifically Cavalier Fighter 18. They are the only exception I'm aware of RAW, and they still only get one reaction per each character's turn. In the scenario described, if a creature attacked someone, the Cavalier used their reaction for protection style, and then the attacking creature used its remaining movement to trigger an AoO, the Cavalier would be unable to make the AoO since they'd used a reaction on that turn. If the attacking creatures buddy moves the next turn (not round), the Cavalier can make any AoO that is triggered, but wouldn't be able to do protection style or cast Shield or anything else that used a reaction that wasn't an AoO (possibly even Warcaster spell casts, depending on your DMs interpretation, since the spell cast isn't an AoO even though it's replacing an AoO. The scenario is so niche that I wouldn't expect much of a clarification on it RAW, but I think think the RAI is clear enough that most DMs would allow it.)
However, anyone that isn't at least Cavalier 18 won't be able to use a reaction again until after their turn as Lathlaer said.
Certain monsters get multiple reactions too, like the marilith, but I think you’re right regarding PCs
Special circumstances aside, Yes only 1 reaction per round (you get it back at the begining of your own turn).