The Classifier is a fixed 90 rounds. If you get a rerun it is one full stage at a time, 30 shots. So take 120 or 150.
A typical club level match will be under 100 rounds in my area. A sanctioned match will be 200 or so.
Four magazines are ok. You get one in the gun and two reloads on the belt; the fourth can be for "Barney Bullets" to get you to an 8+1 starting condition with the minimum of fumbling around.