Keys, Sharps and flats, neighbour keys

The diatonic set of pitches in a given key is seven notes. In C-Major these are:

C D E F G A B.

The key of a-minor uses the same seven notes except when G# is used instead of G for providing leading tone to A-natural. They say that the a-minor is the relative minor key of C Major key and also C-Major is the relative Major key of a-minor.

 

The pitch-set of F-Major is:

F G A Bb C D E

The difference compared that of the C Major scale is one flat ("b"): the Bb note. The keys with only one flat (or sharp) difference are called neighbour keys.

Now take the pitch set of G-Major:
G A B C D E F#. The difference compared to C Major scale is one sharp ("#"). This is also a case of neighbour keys (C-Major vs. G-Major).

The home keys and their number of sharps and flats:

g#-minor, B-Major: five sharps (F#, C#, G#, D#, A#)
c#-minor, E-Major: four sharps (F#, C#, G#, D#)
f#-minor, A-Major: three sharps (F#, C#, G#)
b-minor, D-Major: two sharps (F#, C#)
e-minor, G-Major: one sharp (F#)
a-minor, C-Major: no sharp, no flats
d-minor, F-Major: one flat (Bb)
g-minor, Bb-Major: two flats (Bb, Eb)
c-minor, Eb-Major: three flats (Bb, Eb, Ab)
f-minor, Ab-Major: four flats (Bb, Eb, Ab, Db)
bb-minor, Db-Major: five flats (Bb, Eb, Ab, Db, Gb)