According to this article: Google Sitemaps Updates, Google and the other search engines that make up sitemaps.org are now supporting as many as 50,000 "child sitemaps" of sitemaps index files. In the past, they only supported 1,000. With this increased limit, webmasters can submit up to 2.5 billion URLs with one sitemap index. In addition, the indexed URL count for sitemap files in Webmaster Tools is "more precise", and XSD schemas have been updated to allow sitemap extensions. This allows for the creation of better sitemaps through the verification of more features.