Most affordable mixers have both microphone inputs as well as "line" level inputs or at least mic level inputs that can have their sensitivity attenuated to be able to use them for line level sources. What you're seeing is a combination of mono mic level and also, as in the case of the 3/4, 5/6 number configurations, stereo (left & right) inputs for high level sources such as Ipods, computer audio outputs and tape machine outputs. Usually you can tell how many microphone inputs, and their pre-amps, by counting the number of XLR female connectors on the mixer. The only time this is not accurate is when the mixer is only using unbalanced 1/4" phone plugs for the microphone inputs.
Shop around and try to get something with 6 to 12 balanced microphone inputs for future expansion or just plain redundancy if something should happen to one of the inputs that you're using. Good quality mixers can be found in Mackie, Yamaha, Soundcraft, Tascam, Allen & Heath and many others. It would be better to save up a few bucks more now, than latter having to buy another mixer when the four microphone channel mixer you bought just won't hackett any longer.
BTW, when you see these 12 channel mixers, they're counting every channel, both microphone and line level. So you might be getting 4 microphone mixing channels and the other 8 will be all line level which low impedance balanced microphones will not work into because of the mics very low output levels, unless the microphones are externally pre-amped before hitting the mixer.
Dennis