This is a good question. Also curious about the promotion rules for D.IV into D.III.
256 D.III teams. 64 of those need to promote to fill D.II. Sounds like, on average, 4 teams from each D.III league will promote.
64 teams from D.III are promoting, and a minimum of 32 are demoting. Although I seem to recall a rumor saying their would still be hyper promotion/demotion between D.III and D.IV for another season.
There are 66 D.IV leagues with humans in them. Seems like either half of the D.IV Finalists that didn't win will promote, or a lot more teams are demoting from D.III.