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:
John added that he “wouldn’t be surprised if that particular limit were raised in the future.”
Forum discussion at Google Webmaster Help.