This 5 appearance in Serie A limit? Is that a season or overall? Also, how many teams on average don't qualify for Serie C? Like two? So it will likely be Juventus and Napoli?
1) This year there were 5 teams which didn't make it. Como , Latina , Mantova, Messina and a team i've never heard of Maceratese.
2) B teams will be introduced on request , the criterias of selection betweeen those that request such right has not been shared until now but for sure it has nothing to do with Serie A standings. Maybe it can relate to Primavera standing at max. In addition De La is even agains the B teams anyway so Napoli for sure won't be one of the contenders.
3) It is not only the B teams which will replace the ones who don't make it this year. It is a different level of pick order like the following:
*1st team which doesn't make it will be replaced by a B team
*2nd team which doesn't make it will be replaced by an existing team that has been relegated during the season
*3rd team by a team from a lower league , some kind of lower league (i am not clear on this since after serie C there is amateur football as far as i know so they can't play in pros)
*4th team , start over from B team once more
This kind of picking will be valid only for next year btw...