What about that : Stats by gametrackers , so not all server are tracked .
Counter Strike 1.6 (cs) around 25000 servers.
Counter Strike Source (css) around 12000 servers
Crysis Wars (warhead) around 200 servers
Half Life 1 (hl) around 1000 servers
Half-Life 2 (hl2dm)around 1000 servers
Left 4 Dead 2 (left4dead2) ~ 2500 servers
Medal of Honor Breakthrough (bt) 600 servers
Team Fortress 2 (tf2) ~ 2500 servers
Call of Duty 4 (cod4) 9000 servers
Call of Duty World at War (codww) 1000 servers
Just some ideas .. of what could be added