That entirely depends on what sort of wall you have! Mine is into a breeze block wall so if I remember correctly (it's been a while) they're No.8 x 30mm twin thread screws. If you're going into a plasterboard wall then I'd suggest maybe a toggle fixing plug or one of the metal screw in plugs as the racking on its own is fairly heavy then you have to factor in the weight of all the guns you're putting on there!
Something to bear in mind is that the arms that hold the gun need to be the right length. Mine are long enough to double stack the guns but if you just want one gun per set of arms, get the shorter ones! Also, try and buy everything from one supplier, much like airsoft parts there is some variance in the sizes so for example, the hat stands I have don't fit on the gridwall I have!