Google has at some point quietly increased its sitemaps limit from 1,000 to 50,000. In a discussion on a Google Webmasters forum thread back in April of last year, Google employee Jonathan Simon said that each sitemap index file can include 1,000 sitemaps.
Just recently, however, David Harkness posted to that same thread, pointing to official Google documentation for sitemap errors, which says under the "Too many Sitemaps" error:
The list of Sitemaps in your Sitemap index exceeds the maximum allowed. A Sitemap index can contain no more than 50,000 Sitemaps. Split your Sitemap index into multiple Sitemap index files and ensure that each contains no more than 50,000 Sitemaps. Then, resubmit your Sitemap index files individually.
The larger number was confirmed by Simon, who came back to the conversation, saying, "Thanks for resurfacing this thread as we've improved our capacity a bit since then. The limit used to be 1,000. The Help Center article you point to is correct. The current maximum number of Sitemaps that can be referenced in a Sitemap Index file is 50,000."
As Barry Schwartz at Search Engine Roundtable, who stumbled across this post points out, "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."
In other words, you can have a lot of sitemaps in one sitemap index file. That's some good information to know, and it is a little surprising that there wasn't a bigger announcement made about this.