four of them
In classic Gen 1 fashion, a Pokemon has evolved simply by gathering "more" of itself. And as with Magnemite and Dugtrio, this raises so many questions.
What happens if you catch a single Tandemaus but not its partner? What if you separate a member of the Maushold and raise it independently? Do the small ones grow into Tandemice and pair up, or do they move out to mingle with other Mausholds for better genetic diversity? They're treated as Genderless, so thankfully we don't have to answer some of the weirder breeding questions, but.
They still creep me out.
Overall: 5/10