621

621 = 3³ x 23.

621 is the number of ways to 9-color a tetrahedron.

621 is 4441 in base 5. It is 515 in base 11.

621² = 385,641 contains exactly six different digits.

621 is the sum of two distinct positive pentatope numbers: 621 = 126 + 495.

The USS Haddock (SSN-621) was a nuclear-powered submarine launched in 1966 and decommissioned in 1993.

Source: What's Special About This Number?