Your bench testing should have given you the retention time to achieve the desired recovery of the desired mineral. So knowing your pulp flowrate, the flotation cell capacity and the retention time, you can calculate the number of cells required to achieve the desired recovery.
Example : If we have in total 10 cells for roughing and scavenging flotation, how to determine the line of demarcation so we decide the number of roughing cells and nbr of scavenging cells needed for this circuit?