I lost count, but last count was about 20 mags per AR (14 ARs). I’m pretty comfortable with that and they are a variety of USGI, Pmags, and a dozen or so Lancers and those steel Israeli mags.
They should last quite some time. I did some refurbishing on several 1960-70’s era 20rd USGI and they are more preferred bench mags; new springs, base plates and followers. Granted these are aluminum and those often the most susceptible for having cracked lips, but mine are still looking great.
I do have a few Thermold which are inferior to the construction of the Pmags, but even those are holding up quite well and I suspect Pmags should have a long life-time if not overly abused.