I'm pretty sure there are *lots* of DMs who didn't play with more than 5-6 people. Ed ran 2 campaigns, each below 6 (I think), I ran one at 4, Nick ran one at 4-6. And there were lots of others which I'm not aware of. Brian ran a campaign that started around 4 and expanded to a lot, then splintered and one half died.

8+ party campaigns are not the norm, but more people play in them, and they generate more talk.