You are correct: OpenSprinkler uses software defined master station, which means if you designate one of the 8 as master, you will have only 7 left on the main controller. So you will need an expansion board.
A possible work-around, if you don’t want to get an expansion board is to combine two stations together — if there are two stations that you don’t mind combining together (hence they will always turn on and off at the same time), you can insert both station wires to the same port.
I understand the handling of master station on OpenSprinkler is different from many other controllers: on other controllers there is often a dedicated master station, which is in addition to the 8 regular stations. However, there are some benefits of using software-defined master stations, for example, OpenSprinkler currently supports up to 2 independent master stations and this can be extended further if needed.