How do I manage a sitemap?
To submit sitemaps through Search Console: Make sure that you have verified ownership of all the sites. Create a sitemap that includes URLs from all the sites that you want to cover. You can create a single sitemap that includes URLs from all sites or you can create one or more separate sitemaps for each site.
How many URLs in sitemap?
Break up large sitemaps into smaller sitemaps: a sitemap can contain up to 50,000 URLs and must not exceed 50MB uncompressed.