Very, very few... I have never seen one in any shops I have visited nor do any gunsmiths I know have one. It's a tool very far down the list of tools a smith will buy.
Its to specialized for gunsmiths.
Would be faster using a flat surface and sandpaper to do the work than a surface grinder. It also couldnt be used on all guns. (Very few that i can think of with a perfectly flat reciever)