Category Limit

Is the limit 6? Because after 6 you can't see the button anymore.

I don't understand how categories work exactly. I put an M2 and ammo together in a single category, yet the M2 spawns without the ammo. 

This is because you set the amount of the category to 1. This means only 1 item out of that category will ever spawn. To configure ammo to spawn along with the M2, add it as en extra item. You can do this by clicking on the M2 and adding the ammo as extra item on the bottom left.