To know about the number of developers, read this forum, the mailinglists and have a look at the “recent changes” in the OSM wiki for some month. Then you will see who is involved in developing … which component of OpenStreetmap do you mean especially?
A first entry: http://wiki.openstreetmap.org/wiki/People_involved
some companies dealing with OSM can be found at http://wiki.openstreetmap.org/wiki/List_of_OSM_based_Services#Companies_offering_OpenStreetMap_Services_.2F_Support_.2F_Consulting