When I lived in on-campus accommodation there were iirc about a dozen washers and a dozen dryers for all residents, which was about 600 people. The undergrads at my current uni have the same number for between 4-500 students.
I've since lived in houses owned by my university and the two largest houses had two washers and two dryers between 10-12 people, so much better.
After the first few times, I just started taking people's washing out of the machines when it was done. If they had left a laundry basket or bag beside the machine, I put it in there; otherwise I left it in as neat a pile as possible on a table/bench (never on the floor). Some people really do take the p*ss with leaving their stuff for hours, so I never felt bad about removing washing from machines when it was done.