^^^ I guess I replied but anyways. For interior you need min two brushes but recommended three. One soft brush of 1" diameter, second one of 0.5" diameter for small joints, crevices and steering controls etc. The third one is optional and meant for the carper upholstery. There could be more but these are minimum.
For the exterior, 1", 2" brushes are recommended and one for cleaning the alloy wheels. You could call me up for discussion on what is necessary and what's not.