Scales to Play Over Am9
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 Am9 chord contains the notes A – B – C – E – G. Any scale that includes every one of those 5 tones fits over it — from the root A, that means A Natural Minor, A Dorian and A Dorian Bebop. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
Am9 is also diatonic to 2 major keys: C major (relative minor Am), G major (relative minor Em). 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.