25 million people and still counting :)
Millions.
Call of Duty: Modern Warfare 3 for Wii in only single player.
Two players on one screen. Ten players online.
Yes, there are many invisible, godmode, aimbot, etc players found on Modern Warfare 2. Some even have the ability to infect other players, in turn giving them the mods as well.
By hacking.
No
cap of 9 vs 9 on multiplayer online
activison has not confirmed it yet but many players suspect it as modern warfare 3 is not summed up fully yet
You Cant!
Yes there are many different mods and is commonly done by players.
Special-Ops (2 players max) and Multiplayer (Up to 18 players in a single lobby)
Using different servers.