Too many. 4 in the macpro, 2 WD 640 blacks are raided, one 320 GB stock drive solely for windows and a 1.5tb for backup of documents and video (not time machine).
External is a e-sata OWC dual drive. One is a 1TB time machine into two 500GB partitions, one for my MP, one for the MBP. Other drive is a 500GB seagate for video redundancy.
Then the MBP has its own backup 320 WD drive, not a time machine drive.
Then a 500GB time capsule. Just used for more video stuff, don't trust it as much after reading about all the problems with the power supplys.
I lost 2 HDs at once a couple years back, a system drive and a time machine drive. Had to piece it all my data back from different sources. Not fun. So I am overly protective.