Depends on what you're doing, like most things in this realm.
I usually only record my own material, so I am the only one ever playing. That uses at most two to three channels of preamps at any given time, unless Im recording drums, and in that case, 4-5.
When I record live, I use 16 channels, and in that case, I have two 8-channel preamp units, which were cheap but work well for the situation.
I don't use the pre's on my Yamaha mixer because they frankly aren't that good.
As for mic use, there are an infinite number of setups possible. The drumset can span from a single overhead to two OH's, bass and snare mics, to micing every single drum and cymbal on top of the OH's. You might want to record guitar amps with a close-up dynamic as well as a condenser a few feet off, or perhaps just one or the other.
Backing vocalists can either be singing into one mic or into separate, depending on the sound you're going for (Ive found everyone around one mic gives more of a live feel, vs. individual mics which give cleaner, more controllable mixing options).
As for using preamps for condenser vs. other types of microphones, you always need a mic pre for the microphone, or at least in 99% of applications in a pro/semi-pro setting. A condenser may require phantom power where a dynamic would not, but they both need gain boost to get them to line-level, which is what a preamp does - the addition of phantom power on a mic pre is a seperate entity - it just so happens that phantom & mic pre go very well together as a convenient package.
Any more questions, fire away! Might also want to check the newb section of the forum, as these questions get answered a lot