It can work with any amount of weapons. Basically, if you are in the trigger multiple and press the 'use/evo' button, the trigger_multiple triggers the first target_equipment that only passes if you don't already have that weapon, which triggers a target_AND which only passes if it's gate 2 is enabled (when there is a weapon in that slot). If there is, it triggers:
1) itself, changing gate 2 to off,
2) target_force_weapon which gives you the weapon,
3) func_spawn which is disabled and thus the model disappears,
4) delay which later re-enables the AND and the func_spawn.
Then the next slots are 'checked', and finally you get ammo if you have that weapon.
If you need to know exactly what is happening then set sv_AMPdebug 2, and read/post the server console output after using the weapon rack. Ofc you need to set the inventory/equipment keys to the weapon that you want it to give, not just the model.