this kind of triggers are too hard for the begginers.
anyway here it is....
you need to create a region on the part where your units are, then you use a integer Comparison, then click on (Number of units in (Units in (Playable map area)))
now click on (Units in (Playable map area)), then on (Playable map area) and then select your region.
I can send you a map with that trigger if you want.