Sorry I didn't memorize this information but I was with two major companies. We had extra O2 masks in almost every row, including rows of 2 on the side. I will admit that 737's were our smallest planes but many 4 & 5 seat rows on the bigger aircraft had two extra O2 masks.

The idea wasn't just for lap babies but an extra person were say, in the aisle. There are also some near the doors and in the lavs.

We only had problems when there were two parents with lap baby twins. On one aircraft, we had rows of 2 on the side and two lap babies couldn't be in those but they just needed to either separate (many preferred having two aisle seats) or all placed in the center section.

This wasn't not a big, big issue with seating.