Disclaimer: I have not yet hosted a server and do not know how AI players impact a server's performance.
I recall one of the servers having one bot player in it, which was nice because it let you have something to race against while waiting for other players to join.
Would it be possible for a server to have (for example) when 1 person joins then have 3 bots, when 2 people join have 2 bots, when 3 join have 1 bot, then when 4 join, no bots?
A downside of this would be the apparent false advertisement of the lobby having 3 or 4 players when there are none, but that could be signaled manually in the lobby name by adding a "[BOTS]" tag or something.