As previous posters have said- 50 pax is the magic number. A large number of smaller aircraft - have 50 seats. Not 51 or 52. 50= 1 cabin crew.
its why (generally) aircraft don't have 101, 153 or 204 seats. The bigger the aircraft, the more additional cabin crew makes sense - especially in a multi class cabin.
Once you're into wide bodied aircraft doing long haul, then it's a trade off between service and biz case for the carrier.
The larger the aircraft, the more variance between carriers who trade off legality vs customer service. ie compare the legacy carriers who throw headcount on the staffing vs those who put the minimum number on they can get away with.