[Q]

Mac this depends if you play a turn based or simultaneous game.
In a turn based game you can launch any number of units you want. In a simultaneous game you have to launch all units (at least this was true for Version 1.49. I did not make a simultaneous game with the gold demo yet). Of course there are tricks to get over this limitation if you have two carrier ships or a colony and a carrier ship: load the units you want to launch in one ship and give the order only to this ship and not the colony or the other ship.