How do I create only one instance for create object?

This is how I have it:

System.Score=1:

-> System.Create Object Ball at layer 0 at (Random(50, 450)), 0

Basically, when you score 1 point another ball is spawned from above at random within a range of the play area. The randomisation and it spawning within the set area are just fine, the problem is the entire screen gets drowned in balls from above.

System.Score=1

System.Trigger once while true>-> System.Create Object Ball at layer 0 at (Random(50, 450)), 0

AHA!

Thank you very much