I'll just give an example similar to the example of collision_rectangle from the manual:
So if there are multiple instances of obj_Ball that have entered that collision_rectangle, how do I get the amount that are currently in collision, then add it to a variable named instAmount?
GML:
var inst;
inst = collision_rectangle(50, 50, 200, 100, obj_Ball, false, false;)
if (inst != noone)
{
(My question here)
}