your fan requirements depend on many factors,
Are you overclocking
or intending to at all?
does the pc live in a cubbyhole or under a desk?
whats the airflow in the room like?
does the gfx card have its own fan?
and possibly more factors I cant recall atm
You should be fine in most scenario's with one 120 intake (front or side usually)
and the rear exhaust,
let us know the answers to my questions and well have a better idea of your minimum needs,
fans use a tiny amount of power btw, unless you have a ton of them, even then its no major amount I had 8 fans off one molex line (7x80mm,1x120mm, all led's) then a lightbar set up and my hard drive until recently