Unique buildings | Stellaris Wiki
With the max building slots of 6, only six building can be built in total.
Downloads - Stellaris - ModDB
UI Overhaul Dynamic by Orrie. ... The add-ons will let you increase your building slots, change the font, remove backgrounds, and more.
Unlock More Building Slots UPDATED - Skymods
This mod makes it so a event triggers at the beginning of your game that allows you to add a buff to yourself, everyone, or no one that makes it so that any state owned by the person with the buff has the max amount of building slots unlocked.