I can't tell you how many you should have but I have a friend named "Jack" that bought a bunch of AK mags in bulk from Sportsman guide and they were coming 12 at a time in the mail. My friend has ak mags out the window. And never enough ammo to fill them. 75 rounders are the way to go BTW. the kind that open up in the back but if you get the Romanian one its nice too. Always shoot for under a $100 on those. $10 -$15 for the 30 rounder and $15 - $20 for the $40 round mags.
That is what happened to my friend. There is a whole box under a table that don't get used and just collects dust.