Scales to Play Over Cm9
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A Cm9 chord contains the notes C – D – D# – G – A#. Any scale that includes every one of those 5 tones fits over it — from the root C, that means C Natural Minor, C Dorian and C Dorian Bebop. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
Cm9 is also diatonic to 2 major keys: A# major (relative minor Gm), D# major (relative minor Cm). In a song from one of those keys you can also solo with the key's own major scale or its relative minor pentatonic across the whole progression, not just this chord.