Count units in trigger zone

Is there any way to count the number of units inside of a trigger zone? I want to have a condition that requires a specific number of units inside of a specific trigger zone, and once that number is achieved, then pass a value of TRUE.

For example, I have a trigger zone around a helicopter farp. In the mission, I want to require that a minimum number of infantry be transported to the trigger for a condition to be satisfied.

Is this possible?