FAA minimum is 4 (169 seats with FAA regs requiring per 50 or fraction except AC with 19 or fewer seats) -- occasionally (typically: high loads) there may be an extra FA.

Any time I'm gifting agents/crews I tend to allow at least a 10% (or +2) overage from what I expect -- usually if I don't wind up using it for the person I intend I can make some random UA employee's day a bit brighter (agents working the CSC or BSO particularly). Just remember UA employees aren't supposed/allowed to accept anything more than either $20 or $25 in value from a passenger.

[My wife has alleged that I am "bribing" people despite not expecting/asking anything in return... I prefer to look at it as pre-paying into my karma bank ]