I always ignore that message cause I always thought it says: "You have created the maximum ammounts of units in a game, You will not be able to create anymore units unless some are killed" or something like that.
All Right, I just figured out the maximum ammount of actions you can have with properties. You can only have 64 actions with the create properties. How did I do it you asked? I just created 1 trigger with the action:create unit with properties. Then I just counted as I kept copying the trigger.