Not all teams play it. I think it goes like this:
20 teams from Primera + 22 from Segunda A + 30 from Segunda B, which has 20x4=80. So, the teams from Segunda B that qualify are those who were relegated from Segunda A (4) plus teams qualified for promotion play offs that failed to get promotion (14) plus the next 14 higher qualified the previous season or something like that, which I'm not sure if it's the 5-7th of each group plus the two best 8th or the overall 14 with higher points across the groups.