on what would be a 1-4-7 map, with my scripts, the following would be in play:
1-3 players, one third of the AI spawn sectors are active, and depending on random chance, they get activated and AI will appear. The AI is picked from all available groups in most recent maps. Panhard spawns are less likely (40% of the time) than with more players, and there are less of them in most cases. Smackdown spawns are less likely than with more players and if they happen, there is only one of them (on most maps, a smackdown is a quick spawn of 12 additional AI, adding significant enemy forces very quickly. Maps with Smackdowns have 12 such zones for random locations of where these groups do appear should the randomizer pick them)
4-6 players will see the 1-3 player AI spawns active (because we have 1-3 player trigger going off as well), plus a second set of AI spawns, similar likelyhood, also picked randomly from usually 12 groups of different AI. So now we have 144 possibilities of AI, plus the chance of no AI (so actually 13 x 13 possible spawns). Panhards are now usually 60% likely, and a few more will appear on average. The 1-3 player Panhards are not triggered any longer. Smackdowns are more likely, plus there can be two.
7 or more players: The 1-3 and 4-6 player AI spawns are in place, and we're adding another one. So now we have 13 x 13 x 13 possibilities of AI groups to appear (2197). Panhards are usually 80% likely now, and there will be more. Usually there are 4 different spawns per Panhard trigger. Smackdown spawns can happen even more likely and there can be three, I think. This is the hardest version of a map.
Panhard triggers are independent from AI, so they happen in different places on the map. Timing depends on when a human player walks into the trigger zone. Smackdown zones are also separate and in different places from AI and Panhard triggers.
Regarding your empty map: This happens to us when a server sits idle for many hours before anyone joins the game. It will do that for all rounds on the map that was loaded while nobody was playing. The next map will behave normally. If you then go back to the one that had no enemies, the map will usually behave normal.
It is theoretically possible that some maps will unfold without any AI for quite a while, but that is like winning the lottery. There are very few maps with spawn chances below 60% per trigger and most maps, when you appear with a good number of players will have at least 10 zones that run a randomizer. The one I uploaded today has over 40 such trigger zones and you'd have to be extremely lucky not to see any AI. Plus, Cornholio, a common character at the end of my maps, when present, usually is a 100% likely spawn, so he will always be there, even if no other AI spawns due to pure luck.