I doubt there's a concrete plan. Even the plan when the current season will end was different one day ago, than today.
In general this season segunda b will be having 100 teams (i had read about 4 groups), but next season, the new segunda b will be more compact, 2 groups.
Theoretically this means that a very very small percentage (4 of the 100?) of the teams will get promoted to segunda, while another percentage of the top teams will get promoted/remain to the new segunda b.
The majority will end to the 4th tier, whatever this could be called.