How can i calculate the manpower of a factory?

First identify your plant capacity,counter it against your production requirement,

such that if plant capacity > production req take production requirement else take capacity.

Then identify all activities related to your plants operation,set the standard efficiency for each operation. Identify how many hours would you wish the plant to operate and follow this simple formula 

Plant Capacity or Production Req / (Operating Hours x Efficiency) = Manpower Requirement ;)