The Tibetan administration maintains that there are 6 million Tibetans. This figure is being disputed by the Chinese government, but even by their own data they estimate that anywhere from 5.5 million to 5.7 million are Tibetan nationals. Then again, because you can't do a demographic survey, you can't arrive at an exact figure, so our estimated figure is 6 million Tibetans.
On February 26th, 2013. See this statement in context.