How long is a piece of string?
It really depends on the type of aircraft, the type of operation, the number of units within the fleet and other variables. For example a corporate Boeing 767 might well need far fewer crews than a similar unit flying for a scheduled airline. Similarly a 767 flying for an airline would require crews based on the type of operation. Would there be crews required for downroute placement? Is it a mix of long haul and short haul route flying? How many units are there in the fleet, such that there are economies of scale from a crewing standpoint?
For a charter carrier flying day/ night rotations on a medium/long haul mix in the summer season a figure of 6-7 full crews per unit wouldn't be too wide of the mark, but again it very much depends on the many variables. Those variables not only include the type of operation, but also planning for such things as maintainance, training, leave, sickness, aircraft substitution, etc.etc.