Some people wonder why L4D2 cannot handle more than four survivors at any one time. Well, this video should be more than enough to tell you why Valve decided to limit it. This little clip shows the chaos that ensues when 16 people are allowed to join a L4D2 server, before it decides to crash.
As you can see, anything more than 4 player co-op is just pure chaos.
Left 4 Dead 2: 16 Player Campaign Coop [YouTube]
Left 4 Dead 2: Dead Center with 16 Players [YouTube]
