A cheap way would simply to be to feed the audio out from the numark through a splitter and then send one signal into the mic input on your (or another) pc.. You could then use Audacity to record the input signal and then you would have a recorded copy of your mixes. However, this would not be fantastic quality as you are relying on your laptop’s internal sound card to go from analogue to digital again before you can record it – actually buying the full version of Serato DJ (which you will probably end up doing anyway for other features) would be much higher quality and a lot easier to set up.
Another solution would be if your laptop’s internal sound card supports a “What-u-hear” (also known as various other names) function. My laptop has this and it basically means that any audio you send to the soundcard it presents back to the pc as an input device – you could then record this using audacity at much higher quality as the signal does not have to converted from digital to analogue and then analogue to digital before you can record it..
Hope this helps,
bob6397