Manycraft is a hack of Minecraft (that aims at) supporting millions of players per world.
Some Minecraft maps are as big as Los Angeles. That's pretty cool. But how cool would Los Angeles be with few dozens of citizens ? Probably much less.
Nowaday any server can host a huge map, but most servers can't handle more than few hundreds of players. With an unofficial C++ multithreaded server, running on a dedicated machine with lots of cores, more GB of RAM than you have teeth, a tremendous network connection, a good admin, and fine tuning configuration you may go to few thousands. Which (being the population of my countryside hometown - author's note) is better. But still far from enough for Los Angeles.
Have fun !