Michael Knüppel, "TURKIC LANGUAGES OF PERSIA: AN OVERVIEW", Encyclopaedia Iranica  "Altogether, one-sixth of today’s Iranian population is turcophone or bilingual (Persian and Turkic; see Doerfer, 1969, p. 13).
Shaffer, Brenda (2003). Borders and Brethren: Iran and the Challenge of Azerbaijani Identity. MIT Press. pp. 221–225. ISBN 0-262-19477-5"There is considerable lack of consensus regarding the number of Azerbaijanis in Iran ...Most conventional estimates of the Azerbaijani population range between one-fifth to one-third of the general population of Iran, the majority claiming one-fourth" Azerbaijani student groups in Iran claim that there are 27 million Azerbaijanis residing in Iran."
Minahan, James (2002). Encyclopedia of the Stateless Nations: S-Z. Greenwood Publishing Group. p. 1765. ISBN 978-0-313-32384-3"Approximately (2002e) 18,500,000 Southern Azeris in Iran, concentrated in the northwestern provinces of East and West Azerbaijan. It is difficult to determine the exact number of Southern Azeris in Iran, as official statistics are not published detailing Iran's ethnic structure. Estimates of the Southern Azeri population range from as low as 12 million up to 40% of the population of Iran – that is, nearly 27 million..."
Ali Gheissari, "Contemporary Iran:Economy, Society, Politics: Economy, Society, Politics", Oxford University Press, 2 April 2009. pg 300Azeri ethnonationalist activist, however, claim that number to be 24 million, hence as high as 35 percent of the Iranian population"
Rasmus Christian Elling,Minorities in Iran: Nationalism and Ethnicity after Khomeini , Palgrave Macmillan, 2013. Excerpt: "The number of Azeris in Iran is heavily disputed. In 2005, Amanolahi estimated all Turkic-speaking communities in Iran to number no more than 9 million. CIA and Library of congress estimates range from 16 percent to 24 precent -- that is, 12-18 million people if we employ the latest total figure for Iran's population (77.8 million). Azeri ethnicsts, on the other hand, argue that overall number is much higher, even as much as 50 percent or more of the total population. Such inflated estimates may have influenced some Western scholars who suggest that up to 30 percent (that is, some 23 million today) Iranians are Azeris." 
^2006 Australian Census. NB According to the 2006 census, 290 people living in Australia identified themselves as of Azeri ancestry, although the Australian-Azeri community is estimated to be larger. . Retrieved 1 April 2008.