This should be easy to add to the app / UI (i.e. providing a feature to easily convert back and forth between end time and number of repeat times).
You asked about the maximum current rating: I assume you are referring to how many valves it can turn on simultaneously? Normally all zones run in sequential manner so no matter how many zones you have only one runs at any given time. In this case the maximum current rating doesn’t matter as only one runs at a time. If you want some zones to run in parallel, then it depends a lot on how much current each solenoid draws and what’s the current rating of the power supply. Assuming the power supply has sufficient current capacity, we’ve internally tested running 4-8 zones simultaneously, but we didn’t test more than that. If you want a significant number of zones to turn on at the same time, you should consider using a relay board connected to OpenSprinkler, to avoid overloading the controller, and allowing you to run as many zones in parallel as you want.