While the most one pilot can fly is 8 or so hours, remember that he/she must do alot of work prior to takeoff and after landing. They have to file reports, check the weather, flightplanning......
Anyways, figure that you will need a pilot for every ~6 hours... That makes 4 per seat per day. You get about 30 hours of flight per pilot per week.
Now, you need to realize that there will be always someone on vacation or sick. Therefore, you need backups.
Finally, these guys don't work every day. They have days off.
There is 168 hours in a week
You need 504 flight hours (168 x 3). Now, 504/3 rounded up = 17.
Assuming vaca/sick time, I can see that you'd need 20 pilots per plane. Not too sure on 30.