Nobody really knows, but here is an interesting post on the Official Google Blog…

We’ve known it for a long time: the web is big. The first Google index in 1998 already had 26 million pages, and by 2000 the Google index reached the one billion mark. Over the last eight years, we’ve seen a lot of big numbers about how much content is really out there. Recently, even our search engineers stopped in awe about just how big the web is these days — when our systems that process links on the web to find new content hit a milestone: 1 trillion (as in 1,000,000,000,000) unique URLs on the web at once!

How do we find all those pages? We start at a set of well-connected initial pages and follow each of their links to new pages. Then we follow the links on those new pages to even more pages and so on, until we have a huge list of links. In fact, we found even more than 1 trillion individual links, but not all of them lead to unique web pages. Many pages have multiple URLs with exactly the same content or URLs that are auto-generated copies of each other. Even after removing those exact duplicates, we saw a trillion unique URLs, and the number of individual web pages out there is growing by several billion pages per day.

So how many unique pages does the web really contain? We don’t know; we don’t have time to look at them all! :-) Strictly speaking, the number of pages out there is infinite — for example, web calendars may have a “next day” link, and we could follow that link forever, each time finding a “new” page. We’re not doing that, obviously, since there would be little benefit to you. But this example shows that the size of the web really depends on your definition of what’s a useful page, and there is no exact answer…