You will need about 6 air changes per hour for a commercial bathroom.
First, you need to determine the cubic feet of air in you have in your bathroom. To do this measure the square footage of the floor and multiply by the ceiling height. For example, a 10' x 10' bathroom with an 8' ceiling has 800 cubic feet of air (10x10=100, 100*8 = 800).
Then multiply your cubic feet by the number of air changes desired, in this case 6. (800*6=4800, in the above example
Finally, divide by 60 to get the CFM needed from your exhaust fan. In this example 4800/60 = 80.
So, in a 10'x10'x8' bathroom you would need a fan that moves 80 CFM.
Hope this helps.