There's no such thing as too much ammo.
There's only too much ammo to carry... .
That's why God gave us pick-up trucks.
Or, in the military's case, we carried 7 30rd mags of 5.56 ... that was calculated to get us through contact and a sustained firefight of up-to two hours. Resupply was expected, depending upon terrain, within an hour.
So 210 rounds, per rifle, is the standard prepper's minimum. It can be carried by healthy young males without too much trouble.
In WWII the standard minimum loadout of 30 Gov 06 was only 36 rounds (5x7+1), including the one in the pipe, of 7.62x63. (30-06)
So that's a combat load ... think about that.
IMHO you can never own too much unless you find yourself ammo poor and unable to pay your bills and feed your family.
I figure it in terms of loaded and reloadable. So if I have 500 loaded rounds I want to be able to reload that brass 5 times. Multiply that my whatever exponential multiplier that one might find appropriate and keep going until you run out of brass or components.
Ammo is definitely a precious metal these days and it will continue to be-so for decades given the current political climate and state of the union. It should be treated as a precious metal and therefore viewed as an investment. Therefore, once again imho, you can never have too much.
Without ammo a firearm does not even make a good billy club or rock to throw at attackers.