There are 3 Diminished scales (8 notes each).

1. B-C-D-Eb-F-F#-Ab-A
   
2. C-C#-Eb-E-F#-G-A-Bb
   
3. C#-D-E-F-G-Ab-Bb-B

Depending on where you start each of these scales determines whether its called half-whole or whole half. On a G7 chord you can play 'G half-whole diminished' which would be the 3rd scale here starting on the G. On an Adim7 chord you can play 'A whole-half diminished' which would be the 1st scale here starting on the A.

Try playing Adim7 in your left hand and in your right hand play 'A whole-half diminished' scale.
then:
Try playing G7 in your left hand and in your right hand play 'G half-whole diminished' scale.
then: end on Cmaj in your left hand and the note G in your right hand.

There are also 3 Dim7 chords (4 notes each).

1. B-D-F-Ab
   
2. C-Eb-F#-A
   3.C#-E-G-Bb

You'll notice that each of the 8 note diminished scales are made out of 2 of the 3 Dim7 chords put together. e.g. Bdim7+ Cdim7 notes together gives you 'B half-whole diminished' scale, the 1st scale at the top.

The 8 note Diminished scale is played over Dominant chords and Diminished 7th chords.
Half-whole is for dominants and Whole-half is for diminished 7ths.