IT depends on what you are recording. I find myself compressing some tracks while recording if I know it will benifit from it like bass guitar, vocals and sometimes acoustic guitar if the player is all over the place. Other things I wait to see if they need it when I mix. If you want to compress all 8 channels at the time of recording then you will need 8 compressors or 4 stereo ones. Same goes for mix down if you are summing to a mixer unless you only compress the mix/master bus.

Just a note to remember. If you compress everything all the time you may find you are adding a bunch of noise in the long run as well as killing the "life" of a performance. Use a compressor only when you need it.

What would I need to compress on drums? And what about noise gates? Wouldn't I have to run noise gates on every channel while recording?

You basically need one channel of comression for each channel you need to compress. Same with noise gates.

Some pointers...

* If your not sure if the track needs it, best to leave it alone.

* Compression and gating during recording is discouraged unless you REALLY know what you're going for - Once it's there, you're stuck with it.

* On gates, keep in mind that leakage that sounds good is called "ambience" and amibience that sounds bad is called "leakage" - In other words, the same rules apply - Don't go around throwing gates on everything unless it's called for.