Take the number of duties per week, including SBY cover, and divide by 4. This will give you the required sets of crew required. So if the figure comes out at 5, double it, 5 Capt and 5 F/O's.
Training Capt's are 2 for 1
Current Airline I am working for, divide the weekly duties by 4.198, but that is really knitpicking it.