The whole tone scale is an interesting scale that uses only tones. Using only tones (also called whole steps) in a scale creates an open sound without the feeling of major or minor, since it uses no semitones (half steps) at all.
A wholetone (tone) is together with the semitone the smallest intervals in western music tradition.
In other scales like major or minor, the smaller intervals of a semitone work as a «leading» interval since it has a sound of tension or dissonance. When leading to another note it leads to «resolution».
This is why a tone scale sounds so «open». It could lead to anything!
Since you have to keep using only tones, you will need to use accidentals (sharps and flats) when writing wholetone scales.
Here is a scale with only whole tones starting from C.
The notes used are C-D-E-F#-G#-A# and back to C.
This is 6 different pitches so a tone scale is also called a hexatonic scale.
And here is another scale with whole tones starting from C#:
The notes used are C#-D#-F-G-A-B and back to C#.
Since a wholetone scale uses only tones, and in this way has all the pitches at an equal distance- it doesn’t matter where you begin from. You can start from any key.
So, in a way there is only one, but you can of course start and end on any of the 12 different pitches.
But, in music theory we say there are only two unique wholetone scales. They are also said to be "complementary".
Why? Well, to be unique they need to use different pitches.
So, if you start from C and make a wholetone scale, you’ll get the notes C-D-E-F#-G#-A# and back to C.
Now, if you start from any of those pitches; D, E, F#, G# or A# you need to use the exact same pitches again- so it’s not really different!
But, if we start from C# and make a wholetone scale, you’ll get the notes C#-D#-F-G-A-B and back to C#. Now these notes were not used before! (You could also start from D#, F, G, A or B of course. Still the same pitches.)
Since now we have used up all 12 different pitches we have, there is nothing left to make another «unique» wholetone scale with.
So, we have two «complementary» wholetone scales, each using their own set of 6 different pitches, all a tone apart.
Music using wholetone scales was popular in the beginning of the 20th century.
Composers like Debussy used them in for example «Voiles», creating a soundscape of open harmonies, avoiding any sense of major or minor: