Pretty much all I want to know is how many different possible worlds can the Terraria world maker make. Like in Minecraft you can generate over 18 Quintillion different possible worlds from the generator.
31 Answer
The official Wiki details the use of world seeds when generating a world. With the same seed, world size (Small, Medium or Large), Corruption/Crimson choice and difficulty (Normal or Expert), the worlds generated are guaranteed to be the same.
In other words, any difference in parameter would result in a different world. As the seeds are at most 10 digits, you are likely to have 10^10 * 3 * 2 * 2 = 120,000,000,000 different worlds, which is more than a hundred billion.
