There are 12 tribes, one for each of the sons of Jacob. If I remember correctly Judah and Benjamin are the two non-lost tribes (currently the nation of Israel) while the other 10 are “lost” as in not gathered yet unto Jerusalem.
Now depending on your definition of “lost” they either are truly lost and we don’t know who or where they are (the tribe itself may not even know), or we do know them but they just haven’t been gathered yet for one reason or another. I personally subscribe to both, depending on the tribe. That is my personal opinion and should not be construed as anything taught by any religion.
There were actually 13 tribes, not 12, since Joseph split into 2 tribes: Menassah and Ephraim, and the tribe of Levi exists to this very day.