Already Muhammad recognized the power of numbers:
Narrated Ma'qil ibn Yasar:
A man came to the Prophet (peace_be_upon_him) and said: I have found a woman of rank and beauty, but she does not give birth to children. Should I marry her? He said: No. He came again to him, but he prohibited him. He came to him third time, and he (the Prophet) said: Marry women who are loving and very prolific, for I shall outnumber the peoples by you. (Sunan Abu Dawud, Book 11, Number 2045)
Although numbers do not say anything about truth, cf. More Is Better?, they are an essential aspect in regard to the power of Islam.
In fact, not only the actual number of Muslims in a certain country or region but also their alleged and perceived number can have great influence on policy makers.
In most cases, Muslims like to overstate the their numbers to gain more leverage. Already in 1997 CAIR was speaking of about six million Muslims in the USA. By 2007 they were speaking already about eight million (source) and others, like Sheikh Yasin, are even more "generous" by dropping the numbers of up to ten million (1, 2).
One of the most recent and perhaps most careful studies on Muslim population figures was released in October 2009: Mapping the Global Muslim Population. It gives numbers for all countries, putting the number of Muslims in the USA below 2.5 million. There is no surprise to see that Muslims in turn seek to challenge this report (e.g. here) but it is actually quite close to other earlier research projects (*) which Muslims didn't like either. See also the page Number of Muslims in the United States regarding some earlier claims and surveys.
One thing is sure: Muslims are highly organized and exert political influence far above their actual numbers.