Play the official Alien Swarm maps and any addons through the Offline Practice mode. Alien Swarm:SP also supports a large amount of configurable changes for mappers to help them create new single player maps for Alien Swarm. As the online community who play Alien Swarm seems to be getting smaller and smaller my hope is this mod will revive the game.
Bots are spawned in chambers and brought into play as and when required.
just spawn them ingame when required, i don't get what this is exactly o.O