You'd need one for every mic/instrument level signal you're recording simultaneously (1 for every 2 if it's a stereo pre).
Keep in mind, if you're only recording 2 tracks at a time (of mic-level signals), you'd only need 2 channels of mic pre. Line level signals like guitar processors, keyboards, etc... usually don't need to be amplified because their output is already "line level".
If you're going to record multiple inputs at once, like through a Delta-1010 (8 at once), you'd probably want to get a decent mixer that has direct outs (Mackie, Soundcraft, etc...). This way, you use the mixer's internal preamps to amplify the signal and send it out (via the direct-outs) to your multi-input sound card.
This is what I do, and it's not crap, yet not a Cadillac.