There are many possible choices to do that, but maybe this is a simple one to start with.
Use one integer to store the current amount of ammo on the screen. This can be any name you want, in this example, it is maxAmmo
0 -> no ammo on screen
1, 2, 3 -> ammo count on screen
Then allow launching of new ammo if maxAmmo < 3
Not currently on my own computer to check, but the name for this action might be Int switch
Then you update that maxAmmo value whenever you create new one or destroy one.