To add more details,
you can run as many valve as you wish using external 24VAC power supply, AS LONG AS, the power supply is large enough (VA ratings) to satisfy all motorized valves that are connected to the Terminal 1 and 2 (basically, all of your zone valves, etc.), AND the cables going to each are sized enough to not to cause significant voltage Drop (this is due to all motors kicks in all the same time as soon as C/M contacts gets energized by the Rachio 3 Controller).
Always remember, Motors have inrush current (which is typically 2-5 times of their current ratings) which last only few milli seconds. However they can be a problem if 24VAC adapter you have not sized properly. I always try to recommend higher power 24VAC power source to handle larger loads, such as multiple zone/master valves, etc.)
hope that helps.