combine all keys, select the top face, apply a unwrap and do a normal mapping, give each face a seperate ID then either use Corona multimap or multi sub object to apply different textures and materials to every key.
OR
Take the unwrap to photoshop and manually paint or put numbers in 1 single texture and use that single texture in 1 materials and be done with it.
OR
you should be able to use decals also, it should not be that big of a issue. you can also use align tool to align decals to key centers.