In terms of stockpiling ammo, IMHO, it's just a practical exercise. As several ppl have mentioned, ammo prices keep going up and ammo rarely ever "goes bad." So, why not buy up as much as possible while possible?
During the 2008-2010 run on ammo I decided that having a few hundred rounds of each caliber I had was difficult to track and, in some case, really expensive. So, I standardized on four calibers and now have the option of buying in bulk which is cheaper and ensures I'll never have a bunch of useless ammo should I decide to sell or trade off a certain gun (because I'll have several more in that caliber).
As for how much do I have? Not enough to suit me.
Oh, and BTW, when one gets into competitive shooting sports, the round count can climb dramatically - just like road warriors have "insane" gas or airfare bills, so competitors can have substantial ammo stores.