Audio mixers come in many different sizes and at all price levels. There are several questions that need to be asked to narrow your search to the most appropriate models.

What am I going to be using the mixer for - i.e. multitrack recording, live PA work or both?
What is my budget?
How many sound sources do I have?

As a guideline your mixer needs to have at least as many inputs as sound sources. If you are likely to be buying more equipment in the future you should budget for extra inputs.

What particular mixer facilities MUST I HAVE for my application? i.e. Plenty of EQ, auxiliaries, or Direct outs for recording.

How portable does the mixer need to be?

Will I be doing any location work where there won't be any mains power available?