I come up with 12 please read the following That is a good question the Internal dimensions are 17.5" tall X 13" wide X 13.5" deep so you could fit 2 of these HySkore 6 Gun Modular Pistol Rack in it to safely holds 12 handguns (one rack per shelf) racks are Material
Foam
Height
6 Inches
Length
7.5 Inches
Width
11.25 Inches midwayusa.com/product/537283/hyskore-6-gun-modular-pistol-rack-closed-cell-high-density-foam-black

This is only a dual shelf Pistol Vault. If you are just planing to stack pistols, then it will hold as many as space allows depending on their size and shape. looking at a picture of the interior, I would say comfortably no more than 8 which would leave little to no room for much of anything else. Interior picture here : google.com/search?q=Bulldog+Digital+Pistol+Vault+17.5%22x13.5%22x13.5&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjone6K2c3ZAhXpmuAKHffICfcQ_AUICygC&biw=1920&bih=938#imgrc=uXZNIZEB59GDhM: