As I understand having 10000 players in some area isn't a big problem. Processing players isn't that hard. It seems for me to be much harder to simulate the world around players - flowing water, growing trees/crops, redstone circuits, pistons, mobs. In the worst case players may be located very far from each other, so it's needed to simulate up 10000 world regions. Can this server really do this? Or not really? Or maybe it has very limited world size.