Making an input box is easy, but I don't know if it's possible right now to add a specific number of bots. The problem is, it saves the bots in a file, so they stay across map changes, but they also stay in after you quit and restart the game. If you had it set to add 6 bots, quit, then restarted the game, it would add 6 more, so then you'd have 12...
To be honest, the bots aren't really ready for public use yet. They're just there.