It prevents all the damage that would normally be dealt. So if there are 5 creatures out, it prevents 20 damage to all 5 of them, giving you the prevented life or 100 like you suspected.
December 2, 2017 10:12 p.m.
If Star of Extinction is to deal a total of 80 damage, 2 players and 2 planeswalker then you gain 80 life.
December 4, 2017 12:05 p.m.
To answer the question, shinobigarth has it right. Hallow will prevent all of the damage that Star of Extinction would have done, to anything. So for each creature and each planeswalker than Star of Extinction would have dealt 20 damage to, you will gain 20 life. If there are 5 creatures on the battlefield and zero planeswalkers, you will gain 100 life.