Beware. From the Wiki: "A Mass Driver can send a limited amount of mineral during a year (depending of its rating), but has an unlimited capacity for receiving.".
It means that you can have just one mass driver on Earth, which is able to receive all the packets coming from all the mass drivers at the mining sites in the Solar System.
Suggestion: usually, minerals deposits on Mercury are rather large, so you need several mines and/or automines to exploit them. Much more than 2 AMs.
Then, if mined minerals are more abundant than mass driver(s) capacity to sent them to Earth, they are stockpiled on the surface of the planet/body, and you can collect them through cargo ships (you need also a Cargo Shuttle Station at the body, if it can't be mined from orbit).