That depends on the system you are putting together, but generally speaking you will need at least two available PCI slots for a basic system (Either a Digi 001, and Audiomedia III, or Mix Core, plus an approved SCSI card). However, you can put together a Pro Tools TDM system using up to 10 cards. To find out exactly how many cards your particular setup will use, contact your authorized Digidesign dealer, or a Digidesign product specialist at 1(800)333-2137 or via email: prodinfo@digidesign.com