For the moment I've got about four or so, one for each site. As I'm relatively new to using adsense I don't particularly feel the need to segment the tracking further than each individual site.
I have several websites, so many channels. But basically a channel for each tipe of adsense unit. 3/site
As much as possible.. for every location its own channel, although its not always possible, i try to. Depends on software i use.. i prefer static sites with each article a unique channel..
Too many, certainly more than 16 - and always close to the maximum (can't believe that's a 1000 though) - something between 100 and 200, I think. There are too many, because when it started you couldn't put several channels on the same adblock. So I had channels like this style + position + type. If I had to do it over again I would have far less. But then, right now the channels aren't really for testing anymore, just keeping track of the popularity of sections of my site and their revenue.
I had 3 channels. I'm experimenting with using no channels this week. A friend of mine took out his channels to see what it did, and his earnings went up. It doesn't sound that likely to me, but it doesn't hurt to give it a shot for a week.