i have seen zombie servers with loads of the same bot on them, how do you add more than one of the same bot?

I believe you will have to do this in game, I'm not sure if it is possible from the bot selections before starting the server.
To add bots in team matches, you will have to use addbotx botname x represents the number of the team.
So if i'd want to add Kruger to Alpha team I will have to do the following command: "/addbot1 Kruger".
Repeat this command to get him in the game more than once.
If you are in a deathmatch, rambomatch or pointmatch, use /addbot botname. eg if i'd want to add Commando in my deathmatch game I will do the command: "/addbot Commando"

I hope it helps you out :)

thanks, that helps but is there a way to automate that?

Doubt it, I can give you a little tip though:
press "/" twice to get the previous command you did.

eg, if i did /addbot1 Kruger i could type // and enter and i would add Kruger again :)

COOOOOOOL

*crashes server by adding 100 bots*

Lol, if you add to many I think it just adds this blank black bot and that just sits there.