How many on screen?

Is there a way for the engine to know how many object(s) are on screen at any given time?

I'm trying to do something like

(If 5 blue blocks are on screen)

{

     don't spawn any more blue blocks

     (else)

{

     keep spawning blue blocks

}

}

How could i fake this?

Use system: compare values. If blueblock.count is less than 5, spawn blueblock.

thank you