I don`t think they were told how many strokes they had to give themselves, they just had to make it look like they had put some effort into it. My father is left-handed but because this was discouraged in his day the teacher automatically passed the cane into his right hand and ordered him to cane his left. Although he is able to write with both hands he favors his left for anything requiring strength, he was told to whack himself again if it didn`t look like he was putting the required effort into it!
The school was mixed, my aunts all attended the school and yes girls were also required to cane themselves. This school does seem to have gone through a variety of corporal punishment regimes but it seems punishing girls alongside the boys stayed consistent.