Actually, single births amongst the Ocampans are rare.
The norm is for twins and triplets to be born.
And who says they only have 1 birth per female?
An Ocampan can remain fertile up to 4 years.
That could be 4 seperate children if the Ocampans can regulate a proper birth cycle.
And more children if the births conform to the Ocampan standard.
Kes says something like the first birth is mandatory, because an Ocampan female who has not had at least one child will be incapable of having successive offspring after the first.
They essentially become sterile if they don't have at least one child when the Elogium emerges.
Also, do you want to be the one who tells the Ocampans they're not suitable candidates for Starfleet, simply because their numbers are low?
I don't think Starfleet would discriminate like that or be allowed to!