I remember p@Hobbes's answer to Which countries have built RTGs and used them in Earth orbit and/or beyond? mentioning that the US has put one nuclear fission reactor in space, and that not much was known about Russia's program.
Russia’s Rosatom Corp. has said it plans this year to test a prototype nuclear engine for a spacecraft that can go to Mars. Russia so far has led research in the field and has deployed more than 30 fission reactors in space, according to the World Nuclear Association. China aims to use atomic-powered shuttles as part of its space exploration plans through 2045, according to state Xinhua News Agency.
I'd never heard of the "World Nuclear Association" before today.
Question: So is the answer to "How many nuclear fission reactors have been launched into space?" then 31? The answer to "How many still there?" 31 as well?
note: don't confuse fission reactor with alpha-decay-based RTGs.
above: "A mock-up of a full size Nuclear Engine for Rocket Vehicles Application, or NERVA, in 1967." Source: Bettmann Archives via Getty Images. From Bloomberg