From a lending and an appraisal standpoint, anything over 4 units is considered to be commercial. It is possible that 4 or less could be considered commercial if located in a commercial zone. Even a single family home could have a commercial consequence in that type of setting.
Also, less than 5 units could be commercial when one of the units is a store front, business, etc. Finally, A large single family with many rooms used as a rooming house would also be considered as a commercial.
I hope that this helps you. If you have any other questions, please let me know as I have worked as an appraiser and loan originator as well as a realtor.
Carol Murray Cei