Most of the bots in D.III will relegate naturally. 38 spread over 16 series is a little less than 2.5 per league, and there are 6 potential relegation spots.
Teams that are abandoned have their lineups reset (they use their 12 highest salaried players, with the most expensive starting at PG), so they are fairly easy to beat. And owners are more likely to stop logging in if their team is losing, than if it is winning.
The source of most extra promotions into USA D.III actually come from bots being promoted into D.III (there are 15 leagues that are all bot) and then relegated into another D.IV league as part of the bot cleanup. There are also 11 D.IV leagues with only a single owned team, but most of these will win their league's playoff.
There are 4 unowned teams in D.II which will be relegated to D.III as a normal relegation, and then moved on to D.IV as part of the bot cleanup.
So that makes perhaps 25 extra promotions. Extra promotions are based on place within conference, and then regular season record, so you are not only 16-6, you are a 2nd place team. So I think the extra promotions from D.IV will be 1st place teams with pretty good, but not exceptional records. Perhaps 17 or 18 wins.
In past seasons, 3rd place teams have been promoted as part of the bot cleanup, so D.III is really starting to firm up with experience teams/owners.