I recently received an email from a new AR-15 owner who asked how many magazines they should purchase for their new rifle. I told them that I thought 10 was a good start.
Many carbine courses require that the student have a minimum of 5 magazines and the ability to carry them on their person. This is to minimize the amount of time spent off the firing line loading magazines and to give the ability to practice reloads without having to spend time picking up your magazines between each string. I have found from personal experience that time spent on the line or talking with the instructor is more valuable than time spent jamming magazines with cartridges so I like to show up with as many loaded magazines as possible. In many courses, 10 loaded magazines will be enough to get you through the day. It also nice to have some empty magazines on hand in case you need to download them for certain drills.
In my email response, I just recommended a PMAG 10 Pack from Brownells (Ar-15/M16 Pmag 10 Packs Magpul Ar-15 30rd Pmag 10 Pack). If you are new to the AR-15, picking up at least 10 magazines from the beginning is a great way to put your best foot forward as you train on a new rifle.