According to Nobert Haug there is going to be just 1 extra team with a Mercedes engine. 1 is absolutely the maximum because else the competition would be too stiff. The team that is going to get the engine is unknown as of yet, several teams are negotiating. Mercedes is not going to brand the engine or the team, worse yet they're going to deliver the engine under another name.
The question is how much influence Mercedes is going to get at another team in exchange of cheap engines. Rumour has it
Ferrari has alot of influence in the technical and managment of Sauber in exchange of a cheap engine. Mercedes could also profit from such a deal because they could order the other team to detune the engine.