Any bonus from a garrison applies to units on the garrison's tile and all adjacent tiles. Technically you can have 3 garrisons overlapping in a triangle to provide 3x2 =6 combat strength to a single unit at the centre of that triangle. This should allow you for a very good artillery or air support army wreaking havoc form behind the front lines. However you can not simply spam garrisons to have the bonus apply no matter where a unit is located within that territory.

The stability bonus does stack and a single garrison can provide up to +25 stability with the right civics and tenets.