Earlier versions of OpenSprinkler firmware did have a ‘maximum concurrent station’ limit, but it turns out to be not that useful. So I removed it later.
I don’t think running multiple valves will damage the PSU — first, there is a 2A inline fuse on board which will blow if the current exceeds 2A; also, many common sprinkler transformers are not capable of delivering 2A, so what happens is that when too many valves are open at the same time, the voltage from the transformer will drop significantly and trigger a reset of the circuit, thus releasing all valves.