It is supposed to be only the 7th degree that gives us the fully diminished 7th chord but when I took a closer look today at the 2nd degree of the harmonic minor scale (Locrian 6) it seems to have an enharmonic equivalent of the a fully diminished 7th chord. The only difference is that the bb7 degree is a 6 in the Locrian 6 scale. So even though you spell this chord differently, isn't it still a full diminished 7 chord?
Well, a diminished vii evenly divides the 12-note octave into 4 parts. Therefore, WHENEVER you have ANY dim 7 chord, you automatically have another 3. So in C minor:
vii dim7 = b d f a♭
ii dim7 = d f a♭ c♭
iv dim 7 = f a♭ c♭ e♭♭ (or you can call it e♯ g# b d but then it's not really a 4 chord of c)
vi dim 7 = a♭ c♭ e♭♭ g♭♭ ( or g# b d f)
The 4 note chord based on the second degree of the harmonic minor scale is not diminished. It's m7♭5. Often referred to as half-diminished. But not fully diminished.
Let's take A harmonic minor. from 2nd note - B, D, F, A. Bm7♭5. To be Bdim., it would need the notes B, D, F, A♭. That Bm7♭5 may also be known as Dm6 in a different inversion.
There are actually 3 different dim7 chords like the viidim7 in minor built by 4 minor thirds. After 3 semitones they are repeated - if you consider the black and white keys on the piano.
To each tone of the 12 tones you can build a secondary vii7 dim (like there are secondary fifths in chord progressions). Theoretically there are 12 viidim7 - or even more if you regard the equivalent keys like F#/Gb or d#/eb ... etc.
But practically there are only 3 (and their inversions) to learn.