Quality over quantity, but during Easter I did 6-8 depending on the day. I know it sounds like a lot but I found that if I got up at 7 I could work 8:30-12:30 with a 10 minute break every 50 minutes where I'd do some exercise or have a snack, I would then have until 3-4pm off then do 2-3 hours in the late afternoon and evening. However, you just need to make sure that whatever you are doing is good quality (no passive reading of notes or textbooks, do questions and mark them and all that) and that you are coping with however much you're doing (mental health is priority, so it's about balance to about cramming) and try to have a day off every week. If you were to do under 4 per day, though, I would quesiton whether you would be getting everything that you need to do done!
Good luck!