TBH, everyone who owns a gun should have enough ammo already. Since there is no industry standard, I would say as long as you have enough ammo to load every mag that you have twice, then you're G2G. Just throwing that # out there (a few hundred rounds). Add in if SHTF, ranges will be closed, so it's not like you can go there anyways. It would be purely for self defense or hunting.
Do people expect to be in a gun fight like in A-Stan or Iraq? Even if you did use you're firearm for self defense, HPD would show up and arrest you for 2 days. At best case scenario, they release you pending investigation and confiscate that gun (single) anyways. The question is, would they take just the gun used, or all your guns and ammo?
People stocking up after a mass shooting is more understandable, because prices will rise. So they want to buy cheap while they can or some BS law passes. But for CV, a hurricane, or other natural disaster, no need stock up on more. Let alone pay such a higher mark up. Just wait.