The requirements of a DJ setup are to:
The ability to adjust speed is essential because that is how the musical sources are mixed together. As a DJ it is essential that you have the ability to play two different songs at one time and so you will use two turntables or music sources. The sources of music will be mixed together using a mixer, that will act as the interface between the two sources of music.
The ability to Monitor the Mix is essential and Headphones are one of the most important aspects of your DJ gear. The mixer will allow you to route your input signals to your headphones and "Cue" the music before you route it to the main mix or the speakers.
A turntable has been generalized as a music player that has some form of platter(rotating plate) or jog wheel (spinning wheel) .The function of the turntable is to play a particular format of musical audio and manipulate it in a certain way. You will need 2 sources of music to be a DJ.
This is why most DJ Mixers (mixing Consoles) are 2 channel. DJ mixers can also come with extra channels, but for practical reasons 4 is common in clubs and for mobile setups, because the more channels you have the more audio sources that you can connect. Two channels is good, 4 Channels is great, 8 Channels is cool, and to have more is awesome! The amount of channels or audio inputs you have is a personal preference or a matter of convenience in some cases.