There are several Yor techs that add to the "PopulationCapYor" stat but I don't see any UI elements that show what a planet's population cap is.
I was curious if the planet would stop producing new Yor if it reached a cap so I started a new game, used 'unlock' and 'finish' to get a bunch of high-level production projects, and then queued a bunch (20+) Ultimate Assembly projects. Unfortunately, that crashed the game at the end of turn.