It's not weird mate, just math.
So there are 4 D2 leagues from each of them 5 teams relegate. So 20 in total. In Srbija there are 16 D3 leagues with people in them. So thats 16 league winners + 4best non winners who promote to D2.
4 best non winners are calculated by number of wins > point difference and maybe there are more rules, but I only know these 2.
Cheers congrats on promoting
