Where did the lost Tribes Gone ?

When you do the math you will actually count 13 tribes of Israel. The list contains two tribes (Ephraim and Menasseh) that are descended from Joseph and therefore Joseph is not counted as a tribe even though he was a son of Jacob and brother to the other tribes. That still leaves 13. The tribe of Levi was a priestly tribe and did not receive a portion of the land when they came to the land of Israel (Canaan).
Jacob had 12 sons that produced the 13 tribes.
The Torah (Penateuch) list the number of Jews (counting most males between 20-60 years old) leaving Egypt to be 611,730. One can assume that the total including women, children and elderly to be about 3,000,000 people. That figure grew to approximately 5,000,000 in the time of King David.
The large decline in numbers probably started happening when the Northern Kingdom of Israel was exiled by the Assyrians in 721 BCE. Historians report that the population in the southern Kingdom of Judah (Yehudah) grew in the years after the exile. It is possible that part of the northern kingdom were exiled and part fled to the south.
The tribes from the Northern Kingdom of Israel became known as the Ten Lost Tribes. Many myths were told of how they were in a far-off land, across the river Sambatyon that rained rocks from the sky to prevent anyone from crossing it. Throughout Jewish history there has been much debate and speculation on what happened to these tribes. Did they assimilate or were they intact as a group, strong and proud, able to rescue the rest of the Jewish world that were under constant attack and persecution.
In 586 BCE a similar fate befell the inhabitants of the Southern Kingdom. It was captured and the population exiled to Babylon (Bavel). When the exile was over and Jews started returning to rebuild the second temple there were only 42,360 that returned.
Throughout years of persecution, exile, forced conversions, immigration and poor conditions the Jewish community did not grow at the pace it could have had it been it its own land with more stable conditions. It is unlikely that there are any “Lost Ten Tribes” living together on the other side of the River Sambatyon. It is more likely that the millions and millions of Jews assimilated into populations around the world. There are pockets today of tribes that recognize their Jewish origins even though they ceased to practice the Judaism that they once knew. Every day there are Christians of Hispanic descent that realize that the customs they practice are actually derived from the hidden Jewish traditions of their ancestors. There are tribes in Africa, India, Ghana and Papua New Guinea that claim to be connected to Judaism. As the world becomes smaller we are finding more and more of the lost pieces of the large Jewish population that existed at one time.