Generally as rule we'd work on 5.5 sets of crew per aircraft. So for a single aircraft base, 6. That way 2 sets early, 2 late, 2 days off.
So, for a 738 you need 2 pilots and a min of 4 cabin crew to be legal, so that's 6 lots of 6 so 36 in total. Plus you'll need to allow for base captain and base cabin crew duties. The. There's engineering cover, but that depends how the airline fulfils line maintenance.
As the number of aircraft increases you get back to 5.5 or even 5 sets as you don't need as many standby sets.
Remember you have to have a standby crew as well as an operating crew in order to be fully covered.