There are a number of answers to that. I could have opted for Traktor Scratch and been able to control the virtual decks with actual 1200s. I have tried this interface before, and I do not like it. I dislike the Serato version even more. There are also a mess of hardware controllers on the market that send MIDI data over USB to Traktor. I have yet to purchase one, but there's a number I'm considering. I like the look of the Akai MPD32.
The way I use use DJ software differs significantly from that screenshot. I don't use a software mixer. Nearly 15 years of DJing has given me the irrevocable opinion that there's nothing better than hands-on analog mixing. I have a USB DAC with 8 outputs (4 stereo pairs) and I plug it into a 4 channel mixer. Currently I'm using a Pioneer mixer with rotary faders.
The way I mix, I'm working with the mixer most of the time. I only cue and sync with Traktor. This arrangement works fine until I need to make slight adjustments to a track's speed. My friend Dave has a pair of Pioneer CDJ-2000s ($1800 or so each), which can function as USB DACs and control surfaces in Traktor. Once this new version of Traktor I ordered arrives, I'm going to try out mapping the CDJ2Ks and see how responsive they are. The setup looks pretty sweet:
Picture contains gratuitous gray cat.