Good post. Thanks.

Everyone is going to have a different amount.
Myself, I'm a bit of an Ammo and Mag Whore...  Honestly, I don't ever see me having too many.

For guns you hardly shoot, I'd say 6-10 very Minimum.

For me, Glocks, ARs and guns you shoot a lot, I say build up to 20 or 30 per gun, then once or twice a year, catch a dozen or so on sale.
Once you get enough to fill your normal goto Ammo locker or shelves. Buy Totes...  
Don't even open the Magpul wrappers, just leave them be.  
Then at some point in the future you can either peel off a few to sell and get as pretty good return or use for trade bait for something else you want need.  You don't have to be a Neck Bearder and buy out every store, just catch a few on sale here and there and keep adding.  Over time, It really adds up.

I have a few of those Footlockers you can buy from Cabelas with a tow handle on one end and Wheels on the other.  
I dump all my "Overstock" mags in them for good keeping and quick mobility if I need...  

I've even got a dozen or so .308Pmags and I've never even had a .308 AR...  Its on my list of wants...

Agreed. 10 is a good starting point.

It's like we're playing cards with our sister's kids...

I've got about slightly over 100 30+rounds mags so far and I'm still trying to get more, I agree it's a personal preference.

About half of mine are aluminum USGI type and half are synthetic. Two are 40 rounds PMAG.

I also have some 20 rounds mags mostly are synthetic only a few are metal.

But I have 5 AR so far in 5.56 so it about average around 20 mags per gun.

Plus I've got a Steyr AUG carbine that I converted the stock to use NATO/ M-16/ AR-15 magazines.

If you only have one AR try to get another (or more) as backup and plenty of spare parts for SHTF.

I think its everyones duty to buy as many as can be afforded to make obsolete any future magazine laws.

The bolded part above is something I thought of as well but it they put in place another hi-cap mag ban, my hunch is they will make the ones already around illegal to sell/transfer.

Maybe I'm overthinking this. It shouldn't be more than $200 for 20 more mags.

With that thinking, why don't you own as many AR-15s as can be afforded? They were banned at the same time high capacity magazines were. They are under the same levels of attack that magazines are. Why not own hundreds of AR-15s?

Always have "extras", and don't keep them all in the same place.  Look at what happened to Taran Tactical.  They lost an entire building full of pre-ban mags in a California fire.  They can't buy new ones.  You know, that "all your eggs in one basket" thing.

Good advice.