At least on a AA MD80, the row with 3 seats has 4 oxygen masks. I know this because my son and I were sitting in our nice comfortable exit row when a family of 3 adults with 1 baby and 1 toddler (maybe was legitimately under 2 years old but pushing it) sat down behind me. Both kids were screaming from the get-go. I had a moment of hope when the flight attendant advised them that there could not be two babies in one row due to only 4 oxygen masks and moved the grandmother with one of the babies to the other side of the aisle (poor man traveller who was forced to switch seats as entire aircraft was full). But as soon as we took off, grandma passed the kid back over to the parent in the seat behind me. Kid continues to scream and kick back of seat for entire flight. The flight attendant told them the baby had to move to the other side of the aisle every time she came through, but I guess these people were just stubborn or plain stupid because they continued to ignore her.
At the end of the flight as we were getting our bags, the husband kind of tried to apologize for the children's crying (the baby actually was small enough that I could forgive it--but the older child should have been told in no uncertain terms to stop kicking the seat and be quiet.)