How many of the 12 troll kids would've actually made it decently into adulthood before they were culled or killed by other trolls had the world not ended? Many of the trolls have physical disabilities not to mention Karkat's mutation and Feferi would've gotten slaughtered by the Condesce.

Karkat and Tavros definitely would have died, Aradia was already dead. Terezi is debatable since she is blind, but she can function normally. Sollux might have been culled depending on how he managed his mutation (which made him hear the screams of the soon to be dead). And Fef def. wouldn’t have beaten the Condesce.  So to answer your question, 6-8 of them would have lived.