If the Ruby were to explode it would be roughly the size of the atomic bomb dropped on Hiroshima. Hiroshima was 15 kilotons of TNT. The TNT equivalency of ammonium nitrate is about 0.8, so that would mean that 20,000 tons of ammonium nitrate would be equivalent to 16,000 tons of TNT.

In 2015 and 2020 large scale ammonium nitrate explosions killed hundreds in Tianjin China and Beirut Lebanon respectively. This ship has about 7 times the ammonium nitrate as the Beirut explosion.

The fact that it left Russia is a factor in the concerns about this floating megabomb. I wouldn't put it past Putin to set this off just to kill a few overseas dissidents, with a lot of collateral damage.