I have no idea where this notion came from. A sizeable charity generally has to follow fair labor standards rules and other employment regulations, but I am not aware of any rules that limit the number of job titles that is permissible. The application of the labor rules is governed by the character of the work, not by the title of the job.

 

The question does suggest what fun you could have if you tried to expand the number of job titles to more accurately express the characteristics of the work. Just imagine, a new secretary in a probationary period could be called a Short Paperwork Engineer. After probation, the job could change to Tall Paperwork Engineer. After two years, it could become Grande Paperwork Engineer, and after five Venti. There is no end of opportunity.