I would say 4 or 5. DD had 7 in 2nd year and the mess was constant. Mess begets mess, so just a few housemates dirty dishes dominated the kitchen.
5 in year 3 but a placement year with a dishwasher and all of them keeping a 9-5 schedule, so a routine of cooking and cleaning up at the same time.
Year 4 back at university and in a house for 4. This seemed the best arrangement. It never got too messy, always someone around to chat to and intimate enough to stick to things like cleaning rotas. Easy for everyone to take.one bill to be responsible for.