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At it's current state, there is no method to add / remove bots automatically based on number of human players. It can only be done with custom bot configs and manually adding / removing bots while in game.
I believe that he was working on some scriptable spawning tools (ie. setBotSpawn random), but for now it was something that he had to hardcode in for each map we needed them to not spawn all in one place.
Technically it is possible to do it in the map scripts now that we have ET Pro Map scripting support and scripted entity removal. I suppose we could remove all spawn points on a given map and create new ones. If you had a particular map in mind I can test the theory out.