Hi Maverick,
Can't remember the exact details, but I think that the magic figure is 50, over that and you have to have 2 cabin crew. Its every part of 50, so 2 does up to 99, then 3 up to 149.
I seem to think that some low-costs, like Easyjet keep seats in their 737s to 149, so they can legally only carry 3 cabin crew, but in practice they usually seem to have 4. Hopefullt some-one can correct me, or give more details.
Cheers Eclipse