For classes, I take as many magazines as I have.
It makes things a lot easier if you don't have to reload your mags as often. On the firing line, I also carry as many mags as possible. I only have two mags on my belt, but I have my pockets stuffed with magazines. As I do magazine changes, I move my spares into the main pouch and put the depleted magazines in my pocket or a drop pouch. Of course most of those mags are not empty, so again, they are easier to reload in between evolutions. Every evening, before I go to bed, I load all the mags I have for the next day.
If I could do it, I would take enough magazines so that I never had to reload a magazine during the class. I once took a five day formal class with an AR15 and I took over fifty loaded, 30 round mags.