Pretty much every course description for mags is over-estimated so some dillhole isn't constantly holding up the show. Unless the chest rig is something you've been eyeing and want to try, skip it. Kinda defeats the purpose of the class...using what you got and refining that. Too many dudes waste resources experimenting with crap in class when they should be focusing on other things in class. [Gets off of soapbox]
That amount should be fine for any reasonable class....there's gonna be down time. Don't know if you have a dump pouch but unless you rock sweatpants, you'll have pockets too if need be for the admin sake of it. I'd say it's more important to have a good supply of fresh mags. Better to be that guy with 2 on your belt and a bunch ready to go on the fly than the guy that has 4-5 all on his person and is always jamming mags.
If its some sort of course like a DMR where you could be away from your kit, you can always have a small bag or pack to have near by.