Scales to Play Over C#m(add9)
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 C#m(add9) chord contains the notes C# – D# – E – G#. Any scale that includes every one of those 4 tones fits over it — from the root C#, that means C# Natural Minor, C# Melodic Minor, C# Harmonic Minor, C# Rock 'n' Roll, C# Dorian, C# Dorian Bebop and C# Gypsy Minor. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
C#m(add9) is also diatonic to 2 major keys: B major (relative minor G#m), E major (relative minor C#m). 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.