I have a Kontrol S4. I use 3 channels for tracks and one for a remix deck to add loops/samples. So technically 4-decks? I use the 3rd deck to keep a scratch sentence at the ready and just in case I have to plug something in as a backup.
Using all four could be useful if you’re doing live mashups or even attempting to burn through songs at ludicrous speed! (Don’t know why you’d ever do that though)