I'm looking at stereo mixers. The one I'm looking at has 6 mic inputs (preamps). The first four are numbered from 1 to 4, but the last two are different. The fifth input is labeled "5/6" and the sixth input is labeled "7/8". What does that mean?
they are stereo inputs.

first 4 are mono, last 4 are stereo. you dont have to use them as stereo though, they should be able to be mono as well.