Bob:
I have to agree with you, I know of no minimum number of employees.
Recently, a 'addition' (comm) job got a visit from OSHA, no more than 10 people on the site. Five warnings, one $$ ticket. Next day there were OSHA rules & reg posters/signs, etc. I was not EC on this site.
Also, I don't know if we (NJ) have Fed OSHA, or NJOSHA?? NJ also has a POSHA, for Municipal Properties/Public Employees.
John