This number was most likely added to following the Diaspora as records show communities of Jews traveled and joined or created settlements as far east as China.
Many of the Jews lived in either the city of Bukhara or Samarkhand.
In fact, it was not until an earthquake in 1720 that left Samarkhand uninhabitable, that all of the Jews living in the region moved to Bukhara.
Jews as a religious minority: For the first 10 centuries the Jews lived peacefully as an accepted minority in a predominantly Zoroastrian culture.
While Bukhara was a place of learning it did not reach its pinnacle of importance in the Islamic world until 874 when the Sunnis reclaimed it.