I'm trying (in vain) to get a beginner's grasp of quantum computing, so doing a lot of reading. I've started looking at IBM's QISkit Jupyter Notebooks, and came across the one on MaxCut problems. In there, they give an example of how you could use quantum computers to solve the Traveling Salesman problem, for 4 cities. I may have missed the explanation somewhere in the notebook, but they use 9 qubits to address the problem -- can someone explain why 9? It is a hardcoded value in the actual code, which indicates to me that you could use different numbers...so what would be the tradeoffs with using more or fewer qubits, and how would that scale with the number of cities?
I understand that I can run the notebook and change the value to see how the results change, but I'd like to get a better theoretical grasp of why.