I made it using ReflectionHelper.findField, and it works outside the development environment.
private MinecraftServer server;
@EventHandler
public void onServerStarting(FMLServerStartingEvent event) throws IllegalAccessException, NoSuchFieldException {
server = event.getServer();
PlayerList playerlist = server.getPlayerList();
Field field = ReflectionHelper.findField(PlayerList.class,"maxPlayers","field_72405_c");
field.setAccessible(true);
field.set(playerlist, 15);
}
Is it okay now?