Coming together of 31 planes of existance forms an universe. (At least when it is not collapsed.) Likewise there infinite universes1. Also there is view that each universe contains multiple humanoid planets which is called the human plane2.
Out of these universes there is only one universe a Buddha can appear. This is called the Mangala Sakwala (The special universe or galaxy where lord Buddhas are born). This can be viewed in terms of the number line where there is infinite numbers but there is only one zero. Likewise in the infinite universe there is only one universe conducive for a Buddha to appear.
1Abhidhammattha Pradeepika By Amaradasa Rathnapala, Volume 1, page 27, paragraph starting at line 6.
2 Perhaps based on: Kosala Sutta: The Kosalan
"As far as the sun & moon revolve, illumining the directions with their light, there extends the thousand-fold cosmos. In that thousand-fold cosmos there are a thousand moons, a thousand suns, a thousand Sunerus — kings of mountains; a thousand Rose-apple continents, a thousand Deathless Ox-cart [continents], a thousand northern Kuru [continents], a thousand eastern Videha [continents]; four thousand great oceans, four thousand Great Kings, a thousand [heavens of the] Four Great Kings, a thousand [heavens of the] Thirty-three, a thousand [heavens of the] Yamas, a thousand [heavens of the] Tusitas, a thousand heavens of the Nimmanaratis, a thousand heavens of the Paranimmitavasavattis, and a thousand Brahma worlds. And in that thousand-fold cosmos, the Great Brahma is reckoned supreme. Yet even in the Great Brahma there is still aberration, there is change. Seeing this, the instructed disciple of the noble ones grows disenchanted with that. Being disenchanted with that, he becomes dispassionate toward what is supreme, and even more so toward what is inferior.
"There comes a time when this cosmos devolves. When the cosmos is devolving, most beings head to the [heaven of] the Radiant. There they remain for a long, long time — mind-made, feeding on rapture, self-radiant, faring through the sky, abiding in splendor. When the cosmos is devolving, the Radiant Devas are reckoned supreme. Yet even in the Radiant Devas, there is still aberration, there is change. Seeing this, the instructed disciple of the noble ones grows disenchanted with that. Being disenchanted with that, he becomes dispassionate toward what is supreme, and even more so toward what is inferior.