Miqo'te

[linayar]

It's a just in-case scenario. They don't actually expect anyone to have 10 sons. They barely expect one.

I'm sure it's rare, but being part of the lore, I'm sure it's happened in the past. You don't establish a 10th spot just in case when you barely expect one, and having daughters would not be so rare either way.

At the same time, I think males being rare by comparison should not mean males are rare to the point of barely expecting one. There should be enough to sustain a population of an entire race.