I agree with the "as many as you can afford" concept. However with that said, Glock has substantially increased the prices of their magazines in recent years. Just 4 years ago I purchased 20, 13 round Hi-Cap original Glock factory magazines for my 2 Glock 21's from Natchez Shooters Supply for just $13.99 apiece. Today they are well into the $20+ range for the exact same magazine.
Even with the price increase they, (Glock factory magazines), are still cheaper than most. For example when I purchased my Sig P-226 SCT, (Super Capacity Tactical), it shipped with 4, 20 round magazines. I then purchased a Sig P-226 Stainless Elite. I bought 4, 20 magazines for it at a cost of $54.00 each. Magazines for H&K pistols can run even higher than that. For a 31 round magazine for my H&K USP 9 they want $150.00 for one! By comparison factory Glock 33 rounders can be had for just $35.00 each when on sale from Midway.
Magazines are somewhat of a disposable item. Based on that it pays to have as many as possible, or at least as many as one can afford with the current economy. Also, with the current Democratic leadership, one never knows if or when they'll try to push another Hi-Cap magazine ban. Bill T.