I took the OPs question to be idealistic, so I haven't participated. Is eight a good starting place? Sure, why not. But as Beaverah points out, eight is just a starting place.
But our troop has a reputation of being friendly toward sports and other big activities like band, so we typically experience a 25-35 percent reduction of attendance, depending on the season of sports. I confirmed that figure during January and summer when participation is highest because there is relatively no sports pulling scouts away at the time. In that culture, our scouts plan a head for the activities and experience in the troop. A football player plans to run for PL in the January. We had one scout plan his calendar to run for SPL a head 18 months to fit around his swimming team schedule. We see many players show up late friday night or early saturday after the their game.
We adults figure that the troop can either fight for time in a boys busy schedule, or join them and welcome them when they can. We typically have 95% of scouts go to summer camp, which is an incredible number when you consider 45 percent of the troop is 14 and older. The patrol sizes bounce between 6 to 12 scouts. Ideal? I don't know, we adults honestly don't get involved unless we see some dramatic loss of growth. I'm sure their is some struggle, but with mixed age patrols, the maturity of the older scouts balances a lot of those struggles.
As I said, 8 scouts is idealistic. Real life finds balance through experience. I think the question is where do we adults fit in that circle of life?