When it comes to worldwide, entire internet using population, there is one ultimate source for browser stats. It doesn't list numbers, only a graph, but you easily see what browsers are how large compared to eachother, and their usage trend. I'm talking about the Google Zeitgeist
, of course. In cases other than that, there is no good estimation except for looking at your own logs, or possibly, but less dependable, having a look at the stats for sites with the same content type, update interval, target audience, and surrounding community.