I enjoyed joshu's AMA, and I expect I'd get lots out of other AMAs. But I wouldn't want them to dominate the front page. I think no more than one or two a day. I'm not sure how to ensure that.
I was thinking that number a week, max, myself actually :)
(in realistic terms I suspect the community is generally much smaller/niche here than on HN so there are just not the diverse numbers to have so many AMA's)