lol thats hard when you have 300 units XD
You can't move 300 units, even with triggers, they will bug game. You will need to order small groups of units (10-15) to move instead.
There is no need to have 300 units at same time. This can create really big lag, forget to clean few leaks and game will freeze.
I suggest increased unit stats over number of units.
I saw maps with 500 units placed on map at same time, but such maps aren't so good, as I told already, it's always better to increase hp, armor, damage etc and reduce units number