Is that just domains from which web content is hosted, or just second level domains regardless of whether web content is hosted? Because I can't imagine that there are only 5,500 total .gov domains.
https://18f.gsa.gov/2017/01/04/tracking-the-us-governments-p...
We (18F, me) personally measured at least 26,000 (though some of these are used as redirects or are just error pages, etc.).
