More or less, yeah. It's not a hard limit, and seems like the bigger the route, the higher the limit. But stopping at 6-8 works well as a guideline.
It's hard to get exact numbers, exact details because you don't know exactly what the other airlines have (unless they're willing to tell you to help you test it), because of the various other factors like CI, RI, price, etc that mean all flights aren't requal in the first place, and because of randomness. All those things make your error margins bigger than the change in pax you're looking for. e.g. Say there's a route with 1000 pax. You have 10 flights, there are 25 flights overall. That's 40 per flight, you've got 400 total pax. You add an 11th flight. If the 11th flight is clearly over the limit and changes nothing, you still have 400 pax, 36.4 per flight. If the 11th flight is clearly under the limit and you get full value, you'd get 38.5 pax/flight, 423 pax overall. But if random fluctuations mean only 950 pax fly that day, you get 400 pax again. Plus your flights don't all have the same totals anyway, some might have 30 or 35, others might even reach 45. Which makes it tougher to analyse than it is when you have 1 of 2 flights on a 100 pax route, and then add one to make 2 of 3.