Three is definitely the maximum. I left when the maximum was two per day and that was already a full day of work.
I don't know what the staff and crew schedule is on a three-show day, but the On Camera Audience website listed the first audience arrival time at 8:30 in the morning. There's no indication of what the exit time was expected to be, so I'm going to take a wild guess and say 7:00 at night (that's three and a half hours after the third audience arrival time, which roughly matches the other durations on the three-show days). Audience load-in is only about 15–20 minutes (and there's activity going on backstage while that happens), so moving the audience in and out does not affect the schedule by much in the grand scheme of things (and there would have to be time for loading in and out in the middle of the day when everybody breaks for lunch).
Also, they're not going to tape seven days a week for six weeks in a row.