What WAS said on-screen was this:
KES: The elogium occurs only once. If I am ever going to have a child, it has to be now!
To me that sounds like each Ocampan can only have one child. Which makes absolutely no evolutionary sense.
The other obvious interpretation is that a female at elogium can choose whether to pop the cork on child-bearing or not. If she has a child, she can then proceed with dozens; if she does not, she becomes a drone whose evolutionary niche is acting as dry nanny for the children of others.
The method of bearing the children outside the body might suggest massive production rates: a single child is brought to hatching term very rapidly (the vulnerable external position wouldn't allow for anything else), and then another can be started, with minimal stress to the body of the mother.
As for the polygamy angle, I'd rather think the Ocampa approach would be "life is too short for fewer than two marriages per year". Single births, all with different but consecutive fathers, and with plenty of child care available from those who chose differently at elogium. It's a working survival strategy for some of the short-lived, small-litter lifeforms here on Earth, mainly birds.