Sentinel - max number of times it can proc in a turn?
I'm looking at feats for my Monk. Last night we were fighting the phase spiders and the spiderlings came in. Because we hadn't spread out enough, all of them came towards our group, so they were within melee range of me. I'm wondering if I would've gotten a reaction attack against each one (so... 6?), or if it's a once-per-turn thing. Thanks.
You can only take one reaction per round of combat, so you could only have made the Sentinel reaction attack against one of them. And if you had made a normal Opporunity Attack already, then you wouldn't have been able to make the Sentinel attack. The same applies to any other reaction you might have, like Deflect Missiles.
Reactions take a resource like an action, bonus action, etc do. You can see the resources that each reaction you have takes from your character window, there is a reaction button at the top somewhere. The purple star is the reaction resource and you should find that sentinel takes one of those, therefore you can only use it once per turn.