Simplify Multiple Sitemaps

If you have many sitemaps, you can use a sitemaps index file as a way to submit them at once. The XML format of a sitemap index file is very similar to the XML format of a sitemap file. The sitemap index file uses the following XML tags:

• sitemapindex - the parent tag surrounds the file.
• sitemap - the parent tag for each sitemap listed in the file (a child of sitemapindex)
• loc - the location of the sitemap (a child of sitemap)
• lastmod - the last modified date of the sitemap (optional)
• The following example shows a sitemap index in XML format that lists two sitemaps:

  <?xml version="1.0" encoding="UTF-8"?>
     <sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
     <sitemap>
        <loc>http://www.example.com/sitemap1.xml.gz</loc>
        <lastmod>2004-10-01T18:23:17+00:00</lastmod>
     </sitemap>
     <sitemap>
        <loc>http://www.example.com/sitemap2.xml.gz</loc>
        <lastmod>2005-01-01</lastmod>
     </sitemap>
     </sitemapindex>
  

  Once you’ve made and saved your index file, you can submit your index file to Google as long as you upload and save all your sitemaps to the same location on your host server. You can submit up to 500 sitemap index files for each site in your account.

Source Link: https://support.google.com/webmasters/answer/75712?rd=1