A Google Webmaster Help thread asks if there is a limit on the number of Sitemap files you can have under a single Google account.
Currently, there is no documentation on the limit but there is a 500 limit per Webmaster Tools account.
Google's, JohnMu said, "As it is, the current limit is 500 Sitemap files per account." But John added a way to get around this limit. John said:
You can get around this limit by including your Sitemap files in Sitemap Index files (with 500 websites/account, you could use a single Sitemap Index file per site to cover all of them), by submitting Sitemap files via HTTP ping or by submitting them via robots.txt files. If you decide to use Sitemap Index files, you should still be able to drill down to the individual Sitemap files through Webmaster Tools, so that you can get feedback on them separately.
John added that he "wouldn't be surprised if that particular limit were raised in the future."
Forum discussion at Google Webmaster Help.