As mentioned above, the arrangement of the key signatures in the Circle of Fifths is based on the number of sharps or flats in each key. There are 7 sharp keys and 7 flat keys. However, the top of the Circle, 12 o'clock, is our neutral key, meaning no sharps or flats: C Major.
If you’re talking only about a standard-tuned 6-string guitar played without any extended techniques, then the lowest pitch is E2, and the highest (depending on the model) is about B5.
