I have an idea!

Alright, so lets say you want 9 ultralisks to be owned by yellow at the same time. We will make a 18x18 location that will be able to cover all of them. Now, here for the trigger:
Going Left
Conditions: whatever.
Actions!
Give 3 Ultralisks owned by (yellow) to (red).
Center location (18x18) on ultralisk owned by (yellow).
Give all ultralisks to red.
Give all ultralisks at (18x18) to yellow.
Wait 50 miliseconds.
Preserve Trigger.
In all truthfullness, I have no idea if that will work. But you can try it

. If you want me to, I can a also submit my theory on how to make it go to the right, if you start having that to not work.