An admittedly biased source, but with more or less first hand information, is the Chabad group which just smuggled (or at least claims to have) matzoh from [the] Ukraine to Iran for the Passover holiday. The news article states that "Members of the Chabad organization smuggled a huge quantity of matzot into Iran last week in order to enable local Jews in the Islamic Republic to celebrate the Passover  according to tradition."

"Some 20,000 Jews currently live in Iran. The authorities allow them freedom to practice their religion, but forbid the import of Israel-made products to the country, an act that could cost a person three years imprisonment or the revoking of his passport for five years." 

While they probably didn't get an exact census of the people they were delivering to, I'd guess that they at least got the number reasonably correct. So 20,000 seems to be the way to go.