What have you set up that limits your player to having one shot?
Everything that defines what one can or cannot do is a condition. You're on the right track with variables.
For example, add a variable ShotsLeft=3
Your condition for firing would include System compare variable - ShotsLeft>0.
And your action for firing would include System subtract from - ShotsLeft.
Or you can have an event for after firing, if shots left is more than 0, then reset/reload the next shot.