The number of submitted URLs reflects the number of unique URLs submitted in your Sitemap file, not the number of entries in the file. If Google finds a significant number of duplicate URLs, we may limit the number of URLs that we include for this count. You can resolve this issue by making sure that your Sitemap file does not contain duplicate URLs.
You can use ExperiorTools to test your sitemap.xml. The test will only run each URL once and if you have a sitemap with duplicate URL, only the each first unique URL will be tested.