Thats usually negotiated up front. It depends if you would consider the client making a series of maps with variations your work or his. I.e. would you be able to use the textures and other parts which make up the map again or not. If you make up a map using a texture then you sell him a copy of the map with copyright to it then he owns the copyright to the map. I am not certain that he would own the copyright to all of the textures which makes up the map. When your selling copyright its the artistic composition of all of the elements. So you cant claim the copyright to the colour blue if you purchase the copyright to a painting which had some blue in it. If on the other hand you used free to use elements and textures to make up the map then you may be less fussed about it.