Guaranteed Habitable Worlds
Concept · Stellaris 4.5.2
Guaranteed Habitable Worlds in the Stellaris encyclopedia: This controls how many habitable planets of the same planet class as your homeworld are guaranteed to spawn close to your starting system. Turning this off may mean you will find no habitable worlds near your homeworld at all.
Guaranteed Habitable Worlds in the game's words
The encyclopedia entry as the game shows it in its tooltips; underlined words open their own entry or page here.
This controls how many habitable planets of the same planet class as your homeworld are guaranteed to spawn close to your starting system. Turning this off may mean you will find no habitable worlds near your homeworld at all.
Source: common/game_concepts (concept_guaranteed_colonies) and localisation/english, Stellaris Cygnus v4.5.2 (9776).