Domain tools says:
Registrant Search: "Google Inc." owns about 9,528 other domains
That is about the most inaccurate record there is! :undecided:
Look at it this way...
DomainTool's tacks whois data, stores it etc. So if an email address that I have is "known" to ME or matches the one on file of that domain, then any domain that has a whois record, that matches that email address, it considers Me as the owner.
So even a domain I no longer own, shows I own it, because there is data that shows my email address in whois history.
Registrant Search is basically the same thing and the part that most miss! The number it shows is Whois
RECORDS, not the number of domains.