You can put up to four M-Audio Delta series cards in one computer. Each card shows up as a radio button in the "H/w Installed" box in the control panel tray, so you can configure each card individually.
I have two systems that have two Audiophile 2496 cards each. I also have a system that has one Delta 1010 and three Delta Dio 2496 cards (essentially an Audiophile 2496 card without MIDI) installed. All work fine.