Scales to Play Over A#9
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 A#9 chord contains the notes A# – C – D – F – G#. Any scale that includes every one of those 5 tones fits over it — from the root A#, that means A# Mixolydian, A# Dorian Bebop and A# Mixolydian Bebop. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
A#9 is also diatonic to one major key: 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.