Steve Rockwood at RootsTech around 2013 gave umber of generations we need to go back to have 10 mill

I hope that video caveated the statement that it is an estimate, and likely is based on each generation having an "average" number of siblings who had an "average" number of children each.

How many first cousins you have depends completely on how many siblings your parents had, and how many children those siblings had. I have only 4 first cousins, because my mother had only 1 sibling who had 2 boys and my father had only 1 sibling who also had 2 boys. If someone had parents that each had 5 or more siblings, who all had lots of children, that someone would have 15 or more first cousins.

Carry that logic back each generation and the variance in the number of total cousins is huge.

yes it is totaly based on estimates and averages

but going back 10 generations extreme variances would to a large degree be averaged out

yes a person may have had parents who were only children

but that same sceanrio would rarely continue back many genrations the same

and the BULK of our cousins come not from recent generations - but back 5-10 generations (cousins we connect with at that genration back) - at a time when large families were much so the norm.

So if I had parents who were only children - that really would have little impact on how many cousins I have going back 10 generations - I could still just as easily have millions of cousins 10 generations back even if both my parents were only children. (the bulk of those cousins are cousins who we have a common ancestor at 10 generations back) and not from the few to no first cousins.