"The starting point should be "how many passengers do I need to move?". Possibly , however if you are requested to move passengers to what may be "a location with a restricted size landing area" , i.e. a small offshore located heli-deck , the luxury of a large aircraft S92/189/175 may not be possible , and depending on the numbers to be moved in total and for what specific purpose , then numerous flights may be required to achieve the task requiring a specialist smaller type - in respect additional costs may be able to be charged as well as just flying hour costs.