Each stereo in or out is 1 channel. Windows treats each of these as an audio device (by device, they mean each channel that can be selected as an in or out - not the pci card). The same goes for midi although you are unlikely to want more than 10 midi ports, if you have an 8x8 midi port and dont "Fully" uninstall a previous install of its drivers, Win 2K wont see it on the next install. For exactly this reason, M-audio provide a midi uninstaller too. Just uninstalling in the device manager isn't enough because Windows hangs on to the device details in case it sees it again. All uninstall here does is prevent Windows getting upset when it can't find the device.
Even removing a sound card does not stop Windows from knowing about it and counting it. The same Plug&Play system has to cope with USB devices which may or may not be connected. So to clear all traces of a device an uninstaller utility should be run (after using the uninstall in the device manager). Uninstall means removing every file and registry entry related to the device so a little utility program is the best way to achieve this.
M-audios earlier Delta uninstaller did not do the job properly in Win2000/XP. Get the one you'll see by ticking the "Show Beta" in their web sites drivers search. Or, while your at it, get the 29x12 driver (it includes the uninstaller). Although it's listed as a Beta version, 29x12 really is the best Delta driver so far where multitrack recording is your primary use.
I've gone to the trouble of typing all this and you still don't say what Windows you're running !!!