You need as many channels as there are singers and instruments that need to go through the PA. So for most bands, an eight channel mixer will be enough. As well as microphones, you may want to plug in an acoustic guitar, keyboards or even electronic drums. Small venues (local pubs, for example) don't require drums to mic'd up, they're quite loud enough! But if you play larger venues you might need to mic the bass drum and snare (or more) so think about future-proofing by buying something a bit larger. Sixteen channels will usually be enough even with drum mics added.