Sentinel - max number of times it can proc in a turn? - eviltoast

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.