I store most of my surplus in military ammo boxes with desiccant packs inside
I'll second this method.
As far as how much, usually around 500 shotgun shells (12, 20, and .410), about 1000 .22lr, and 300ish .45ACP
I have a mild interest in guns. Actually, I think the clinical term is "obsession," but that makes me sound like some kind of gun-nut. Which is fair, since I am.
Wastin' away my future children's inheritance one box of ammo, range fee, and bottle of Hoppe's #9 at a time.