Assuming Matt's number of 1.8-2.0 million is correct (the IRS says 1.4 million had done so by September 17--see http://www.irs.gov/newsroom/article/0,,id=213375,00.html ), I'd guess the actual number might be in the range of 1.3-1.4 million.
The large majority of the 1.8-2.0 million will have been renters. However, that doesn't take into consideration the number of college graduates (and some high school graduates) who might not have previously been classified as renters. According to the Bureau of Labor Statistics, an estimated 1.24 million bachelor degrees were awarded in 2008. See http://www.bls.gov/opub/ooq/2000/Fall/art01.pdf So, add in a few more who graduated from graduate school, subtract about the same number who probably went on to graduate school, and throw in some high school graduates, and you have an additional population of at least 1.25 million eligible buyers. True, a lot of them will rent. Some will return home to live with their parents. But let's say that 10% of them buy. Maybe they saved the money themselves. Maybe their parents helped with the down payment. (Anything to keep Junior from moving back in!) Maybe after 21 years they were just tired of living at home, living in dorms, etc., and finally wanted a place of their own. I'd be willing to guess that at least 125,000 college graduates (and probably closer to 200,000, maybe even higher) purchased a home in 2009.