Estimation for World Centenarian population around 450,000
Actually, I was being silly before with listing 500 as an answer.
The actual number for USA only was
projected in 2000 as 72,000.
However, these numbers vary wildly as the following link is showing a guess-timate of 55,000 in USA is for 2005.
http://en.wikipedia.org/wiki/Centenarian
I quickly did a search that you might want to try:
Type the keyword or keywords in a search engine such as Google or Metacrawler (searcher of all search engines).
Keyword is 'Centenarians' or 'world population of centenarians'.
One such search resulted in this link:
http://www.grg.org/calment.html
"As reported on the front cover of USA Today of August 24, 1999, The U.S. Census Bureau has forecast that the number of Americans aged 100 or older will increase by more than 22 times the 1990 estimate of 37,306.
In October 2001, the US Census Bureau actually reported that there were
50,454 US centenarians (a more reasonable 35 percent increase) out of a total population of 281.4 million Americans."
"From present data, the number of worldwide centenarians is around 450,000. However, if one considers only the total number of supercentenarians (100 plus), this number falls dramatically to an estimated 300 to 450 worldwide, of which only approximately 80 are validated."