Используйте оператор modulo:
In [3]: notes = ["a", "a#", "b", "c", "c#", "d", "e", "f", "f#", "g", "g#"]
In [4]: len(notes)
Out[4]: 11
In [5]: note = 11
In [6]: notes[note]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-6-707e7e351463> in <module>()
----> 1 notes[note]
IndexError: list index out of range
In [7]: notes[note%len(notes)]
Out[7]: 'a'
In [8]: notes[note-11]
Out[8]: 'a'