You mean there are DD's listed on the maintainer list for more than 500 packages.In the past I don't believe any person can maintain over than 500 packages by oneself. However when I'm actually analysing Sources.gz from Debian Mirror, astonishing things emerges. There is really DDs who maintain over than 500 packages in main section.
In the past before "team uploads" were formalised it used to be common practice to add yourself to the maintainer list when uploading a team-maintained package you were on the team for but not on the maintainer list for. Even if you had no particular interest in that package. People working on getting large transitions to go through can end up touching a lot of packages.