On page 14, middle column, it says "Once per Turn, when you spend Intent to pay for any action, you may decide..."
I'm like 90% sure "you" means "each player separately" and not the collective "all players as a group" but just to be totally clear... boosting intent could happen 5 times in a single turn, if every player in a 5-person game did it...right?
Yaw, it's per Turn per character. Confirmed here: https://boardgamegeek.com/thread/2542643/article/36310778#36310778