The PCJr emulates about 6 mappers natively. But that isn't an accurate representation of what it does. It may emulate only 6 of the hundreds of mappers out there, but that still represents 95% of all NES games, and about 75% of Famicom games.
I doubt that the Famicom MGD1 emulates as many mappers as the PCJr, but without any hard data about that system, we can't know for sure.
Technical info about the PCJr is still available here: http://mypage.direct.ca/c/ccovell/nesdev.html