There's no disdvantage that I can think of. And 20,000 urls is well under Google's stated maximum:
Q: How big can my Sitemap be?
Sitemaps should be no larger than 10MB (10,485,760 bytes) in length when uncompressed and can contain a maximum of 50,000 URLs. This means that if your site contains more than 50,000 URLs or your Sitemap is bigger than 10MB, you must create multiple Sitemap files and use a Sitemap index file. You should use a Sitemap index file even if you have a small site but plan on growing beyond 50,000 URLs or a file size of 10MB.