CL60 hit on it, but try the NBAA survey. It gives you two formulas based on either days available to be flown or hours flown.
Extrapolated from that formula if the airplane is available to be flown 180 days per year or less you need 1 crew. Or if you fly less than 200 hours per year 1 crew is all you need.
For a 135 example, the airplane is available for 330 days (365-35 day’s mx) per year using that formula you need 2.5 crews (i.e. 3 captains, 2 copilots). Or flying 700 hours per year you also need 2.5 crews.
Of course these formulas don't mean squat in the 135 world. They'll fly with as little crews as they can to cover their trips. I can remember putting in 110 flight-hour months being the only Lear Captain for a sleaze bag 135 operator.