I recently installed a DC controller, replacing an older PC controlled unit. All the relays and valves are designed for 24v AC.
It is now working just fine and currently runs three valves and a pump relay concurrently with no problem.
It seems inevitable, however, that at some time in the future, we will suffer from a drought once again, and will have to live with restrictions on our watering days. When that happens, I will probably want to increase the concurrent operation by at least one more valve. I have not found any information about what the maximum number of concurrently operating solenoids is. I am supposing that if the hardware and the software are not stressed, the power supply might be.
The software does not limit the number of concurrent zones. Generally when a zone is running, it draws somewhere around 200 to 300mA current. So you can multiply that by the number of zones you want to run simultaneously to get the minimum current rating of the power supply. I think if you plan to run 3 zones at the same time, the 1 amp power supply that comes with OpenSprinkler DC is sufficient. Beyond that you may want to upgrade to a 1.5 amp or 2 amp power supply.