If a site has 2 million pages, how many should be included? The limit is 50K... So what happens to the remaining pages?
Just split your sitemap in multiple sitemas and put a limit per sitemap of 10-20k links this way its easier to control the links.
Generally sitemap file is of 10MB of 50K urls. So You can create multiple sitemaps and link them within a sitemap if you got more urls.