While the firmware doesn’t limit the number of valves that you can turn on at the same time, I would say that simultaneous zone runs should be limited to 4 or less. Running a lot of zones at the same time can significantly increase the current load of the system. If you do want to run many zones at the same time, adding an external relay board would be the best since you can then use very thick wires to allow much more current to flow through it.