tModLoader
Terraria with tModLoader
Single-Threaded Game
Terraria is single-threaded. Adding CPU cores won't increase player capacity. Clock speed and reducing world size matter.
Mods Must Match Client-Side
Players must have the same tModLoader version and the same mods enabled (in the same load order) to join.
Startup Tab Overrides serverconfig.txt
World, slots, password, language and world size set in the Startup tab override what's in serverconfig.txt, so don't waste time editing the file for those.
Official Documentation