In the few games I have attended this year, I've rarely had to carry more than one or two additional mags, depending on the replica I am using (Bizon for example is a bit more hassle to carry a spare than a kalash). Due to the nature of REC games with multiple reload times and that I have usually stuck to semi only over the years and a speed loader or two usually lasts me long enough in those situations.
Of course, that has backfired on me before when I neglect to carry a spare magazine and the current game lasts longer than expected.
If I had to be fully loaded for some reason it would probably be between 5-7 magazines,