How do I calculate the proper fan size for a bathroom exhaust and the number of pickups?
The usual rule of thumb is to equate square footage to CFM.  

7x8= 56 SF => 56 CFM.  Rounding up to 80 is typical.  

Y'd pickups are nice.  

The ceiling mounted Panasonics are plenty quiet, though (meaning just 1 pickup).  I prefer exhaust fan closest to (or in) the shower (new Panasonic with recessed light is very nice) and connected to a countdown timer.

Be sure to have any shower electrics run from a GFCI.