Unanswered Question - SPOILERS!!!!!! - Potential Plot-hole

In the first scenario, the teacher explains that the bunker they're trying to get into, only has enough oxygen for 10 people to last one year - the duration the survivors would be locked down until the end of the disaster. Here, they eventually let in 9 and close the door.

In each situation, the main objective is to repopulate the planet which would imply that more than one child would be born. Given a baby takes 9 months to develop & arrive on this Earth (give or take), would multiple offspring not drain the allocated Oxygen levels to the point where the adults could not survive until the door could be opened...

I would assume the same oxygen point is applicable to scenario 2 & 3, however I don't recall if the level of air was mentioned or carried over. Again, if the aim is to populate the planet as quick as possible (example - Petra looks to sleep with the lecturer almost straight away) then this wouldn't be sustainable?