How many endangered animals are there in the world and what is the most endangered of each animal group?
A cursory search yielded this site, which appears to be a central repository for just about anything you’d ever want to know about endangered or threatened species. You can probably start with the “Endangered Species Lists” on that site, and go from there.
From one of the many links available from there, it appears that the Black-Footed Ferret may be among the rarest of (wild) mammals, for starters.