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Do I need two turntables to scratch? What exactly do I need two for?

From what I see, DJs use one for playing music and the second to scratch. Im not going to get into DJ-ing too much, just for fun and kicks. My question is, can I use ONE turntable with my Macbook and scratch to the music playing for my Macbook? I see Mixers that include docking stations for Ipods. So again, from what I see, it looks like theres music playing all the time and then only one turntable is used for scratching. So would I be able to mix my own music on my laptop and then scratch to it using my solo turntable? Also, if I do only need one turntable to scratch, would I even need the mixer? Thank you.