I'm gonna guess you've never ran a server. A single survival server (like this is) rather than a big linked network of minigames with bungee cord, even on the best hardware available & an optimized minecraft server (spigot, paperspigot) starts to seriously struggle well before the 250 players online mark. You can squeeze out a little bit more by nerfing some core game mechanics (limiting render distance, redstone, entity spawning and tick speed) but that's it. Keep in mind that the game is mostly CPU bound and has little in the way of multi-core threading. The bottleneck then is single core CPU performance (a good amount of ram and a fast SSD help a bit but only so much). While you can of course have 1000s of players so long as only 250-300 of them are ever online at the same time no pure survival server is going to be doing anything like 2 billion players.