I am not sure when this happened, but fairly recently, Google has changed the number of Sitemaps you can reference in a Sitemap index file. The number use to be 1,000 sitemaps can be referenced in a Sitemap index file, now the number is 50,000 Sitemaps. This is a huge increase in capacity.

Still, each Sitemap file can contain up to 50,000 URLs, so technically 50,000 multiplied by 50,000 is 2,500,000,000 or 2.5 billion URLs can be submitted to Google via Sitemaps. That is if I can add correctly.

Googler, Jonathan Simon, said this in a Google Webmaster Help thread:

Forum discussion at Google Webmaster Help.