Hmmmm... to take a guess (being safe that is), typically you would have 2 or 3 4pin molex on one rail, which would can hook up 2 HDs and 1 DVD/CDROM. Basically that would be around 20-30W per HD or DVDROM.
Soooo... 60-90W roughly on a single rail. If you have fans that are rated 3W a fan, to equal 60W that would be 20 3W fans.
. o O (just the thought of running 20 fans)