How do i set a maximum of sprites?

I'm new in construct2 and i'm trying to make a little upgrade game.

Well this is the Example: A ball creates every 5 seconds, but i don't want more than 4 balls in the scene.

That's all Thanks~

<img src="http://i.imagebanana.com/img/o7sfk5s7/code.png" border="0" />

There is an expression to count object of specific type.

+Every 5 seconds

+Ball.count < 4   -> System: Create object ball....

<img src="http://i.imagebanana.com/img/o7sfk5s7/code.png" border="0" />

Thanks! I understand now :D