redwolf has an great selection of MP5 mags. Myself I bought the 10 mid cap pack and although they are plastic they hold 120 rds.
Myself at a milsim might go threw whats on my vest in an entire day. 5 Mid (120), 4 Low (50) and 2 Real caps (30).
But thats in a 24hr game. If its just skirmishing then maybe a mid cap or 2 depending how much cover Im giving out.
Other then that all I can say is do not get hi caps. Horrible horrible invention they are.