I always look at the price of mags for the particular gun I will be buying as do you I'm sure. A cheap gun is great but when you have to pay $30+ for a mag you may have well got an expensive gun with cheap mags.
I'd say seven is a good number just because most tac vests hold six and that will leave you one in the gun to rotate.
I have at least 35 for my 91 just because they are so available and cheap. I always try and buy factory mags because they are usually most reliable. Whan at shows or a store I may pick a mag up here and there to slowly stock up, I agree with however many you can afford.