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.
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.