You can add the number of bots you want by editing these parts of the .ini:
[UnrealGame.UnrealMPGameInfo]
BotMode=0
MinPlayers=10
[Engine.GameInfo]
GoreLevel=2
MaxSpectators=2
MaxPlayers=18
AutoAim=1.000000
GameSpeed=1.000000
bChangeLevels=True
bStartUpLocked=False
bNoBots=False
But the easiest way is to add them using the Web Admin Interface, then you can chage the numbers on the fly, if you need to.