I just bought a 24 gun safe. The number they tell you, i.e. 24 is long guns.
I don't own that many long guns, so they are one one half of the safe, and the other half is handguns, ammo, holsters, paperwork, passports, etc.
I wish I had bought a bigger one, as it's about full and I only have 7 guns in there.
I'm going to build a few more shelves to go up higher for the paperwork, but I'd suggest buying one at least twice the size you think you'll ever need.