Regarding how many persons of the opposite sex might work out, 1 out of 5, one out of 10, 20, etc. . . .
Think of the situation folks have faced over hundreds of years, particularly in small cities.
e.g. In a small farm community there may have been only 3 girls and 3 boys in a particular grade.
And often these folks paired up into 3 marriages that lasted for 50 + years.
I have seen it in my Grandparent's generation.
I have seen cases where children in one family married children from the next farm.
They made it work, because there was no alternative.
In today's world, it is possible we have too many choices.