And a lot has to do with how much other stuff is in the sauce.
I just made some BBQ sauce with 6 Douglahs in 10 cups of sauce. I thought that would be enough, to really kick it in the bhutty, but it really wasn't that hot so I added some Bhuts. 6 Douglahs in 5 cups of sauce probably would have been better. Still not as nuclear as some like it. First time using that recipe and peppers, so all in all, I think it turned out well.
The kid made a fruit based sauce with peaches and peach habanero, 2 1/2 peppers in 6 oz of sauce. That's a really nice heat level. Also made a lychee w/ fatalii with 3 fatalii in 6 oz, and that again is a really nice burn, but you can still taste the sauce. For both of those recipes, we started out with about 12 oz of stuff and it reduced down to 6 oz.