I'd rather have the script find by itself the prim number of the seats: by giving the seats prim meaningful names (e.g., "seat 1", "seat 2", etc), it's quite easy to do and then you can unsit avatars that don't sit on the right spot, or avatars in excess. It also ensures that should the vehicle be modified (prims removed or added, vehicle object prims re-linked), you don't break the scripts in it...