36 building slots
Unique buildings - Stellaris Wiki
A bonus building slot at the start can kickstart your early game, but you have tons of ways to get more building slots by the late game.
MapleStory Gun - AyumiLove
35 – Maple Gun ATK 36, Effects: Avoid +10, 7 Slots, Fast Speed.
Unlock All Building Slots - Skymods
Read more about Unlock All Building Slots at Gameplay on Skymods.