I don't think there is a "usual" about such a post.  The number of people such a person would supervise would depend upon the size of the company and how it is structured.  Some companies and organizations may work on the principle of X-number of supervisors to Y-number of supervised or, as in my company, the number of people needed to do a particular job.  

You may get a more useful response from other DD posters if you provide some more information: the industry you work in and the location (not necessarily precise location, but something along the lines of "Tennessee" or "England" would be help).

Best wishes,

Raindance