Several lifetimes worth, but I'm unsure if there's such a thing as too many in the FDA's America. But only 5 or 6 in rotation... until there's 10-12 of 'em, because I like playing with stuff in my stockpile
Then, eventually, I compress it down to 5 or 6 in rotation, before it grows again.
The thing with mods is... I don't need 1.2gigawatts, and all the high-end mechs from 2010 still work exactly the same in 2017. I started with junk in 2009 (blu) and then never went back there, so stuff accumulates, 'cuz it doesn't really break
Anything that goes wrong with a mech can be fixed with O rings and parts available at most hardware stores. The only regulated mods I have that survived from back then, are ProVari's.
Tanks are another story. Over 200, but I'm not ever gonna use 125+ of 'em again, unless the world is ending and there is literally no other option. Tanks have advanced very rapidly and only stabilized on overall design principles after everybody got "inspired" by Kayfun and workable RTA's arrived around 3 years ago; and in my subjective opinion really good tanks that I enjoy have only been on the market for about 18 months. I don't do super high wattage vaping, but I don't really enjoy the sucking golfballs through a straw super-restricted MTL experience; much prefer RDL airflow, which is relatively new.