Best way to keep the bots on the server is staying on the same map forever. Bots don't like map rotations. If COD4 behaves like COD2, the game engine will allow 32 bots. When rotating maps, the old bots will get stuck in spectator mode, and new bots will spawn (up to 32 max). Kicking the bots doesn't help. The game remembers the slots occupied by the old bots. New bots will spawn until there are (were) 32, or your server is full, whatever comes first.
No Mbots for X4, but I think ATB has been working on getting the PezBots in X4. Not sure about the status.