Well, the population of Westeros is around 40 million, and medieval European societies tended to have around 5% or so of the population as the nobility, with some as low as around 1% and some as high as 10%.
That makes sense even in the modern US with the 1% vs 99% stuff that’s still 3 million out of over 300 million people. In Westeros when you say highborn you are not just talking about the Lannisters, Hightowers and the Arryns. You are talking about the Osgreys, the Smallwoods, and Podric Payne who:
was born a member of a lesser branch of House Payne. His father was a squire to richer cousins and his mother was a chandler’s daughter. He was still highborn enough to save him from Tywin’s noose.
Yes. And just as today, there’s enormous amounts of stratification between the top 9% and the top 1%, and the top .99% and the top .01%, and so on, you’d have the same thing with the nobility with around 100 people in the Great Houses at the start of the series (so the top 0.0000025), who are above a few hundred or a few thousand principal lesser Houses, then probably tens of thousands in all the minor houses, with the overwhelming majority of the nobility being landed knights, household knights, and hedge knights.