The numbers refer to the number of inputs and outputs a mixer has.

The first number tells you how many inputs are available. It does not specifically state how many are mono or stereo, so be careful. Mono inputs tend to have more features and a more comprehensive EQ than stereo inputs, so you should check a more detailed spec. Mono inputs tend to get used for Microphones and DI boxes, and the stereo inputs will be used for CD, cassette or tape machines, as well as stereo keyboards. For example, the Soundcraft Folio 12-2 mixer we have has 8 Mono inputs and two stereo inputs, so would not be appropriate if you need to use more than 8 microphones.

The second number (the '4' in the example above), tells you how many 'Group' outputs the mixer has. Inputs can be assigned to these groups which generally have a single fader each. You can then either use this fader to control the groups own separate output, for example, to drive a separate centre cluster of speakers, or to control the level of a group of inputs, for example a drum kit, which are then passed to the main stereo output faders.

The final number (the '2' in the example above), relates to the number of master outputs. This is usually in stereo (i.e. 2) or sometimes allows for a stereo and a separate mono output (which is often used to drive the aforementioned centrecluster) in which case the final number would be a 3 or even '2+1'.

So, in the above example, the mixer has 24 input channels, four groups and 2 master outputs.
Remember, that these numbers do not generaly include effects returns on the input side or auxiliary and matrix outputs on the output side.