3 Words: Unit of Fire.
Determine what your unit of fire is, per caliber, then determine how many engagements you need to plan for.
For me, 210 rds of .223 plus 30 rds of .45 = one basic rifleman loadout. Figure 3 .223 shooters in a fireteam, so now you have 630 rds of .223 and 90 of .45ACP. Add in 140 rds for one DM loadout, plus his pistol, and now you have a complete unit of fire for a fireteam.
Add in some home defense projections, using 12ga mostly, or perhaps a pistol carbine caliber, and away you go...
For me, its 3 units of fire all the time, and these rounds are not for training. For training, I maintain 6 months worth of training rounds, averaging 500rds a month in all calibers, but .22 - that runs another 250 by itself. So, you can see, that hoarding tens of thousands of rounds is unecessary, unless you believe that supplies of ammunition will be zero'd legislatively.