I'm reading Programming Quantum Computers trying to understand Shor's algorithm. I learned there that we prepare a state $|x^i \bmod N\rangle$, then apply the QFT to this state. The QFT changes the amplitudes from a uniform superposition to large amplitudes evenly spaced out by the period of $x^i \bmod N$. For example, here's a graph of the amplitudes after applying the QFT with $N = 35$. That's on page 241.
The book says there are 12 spikes evenly spaced. I see a lot more than 12 spikes evenly spaced. Should I count just the highest ones and stop when I've counted 12? But isn't that subjective? How would I figure out that the number is really 12 from just looking at this graph without knowing the right answer? (In other words, how do I get 12 out of this?)