It's a tricky question, I'd say. Same with the mp3 widget.
Let's say we count it as "one" ad...then common sense tells me that a 400-word article with 8 Amazon widgets looks spammy. Even 3 or 4 would be too much, no?
It's a matter of balance and attitude. We don't want to create product listings, landing pages, or thinly-veiled company ads. We want to write articles that are useful and attractive in their own right, even without ads.
Advertisements should be the icing on the cake, so to speak...an additional tool to give to the readers what they've been looking for.
One or two ads are sometimes much more effective than a whole bunch of "buy-me's". Fewer is better because it doesn't distract from our story.
The story sells, not the ads. And once the readers have clicked through to Amazon (Zazzle, AllPoster), they'll find a huge selection of choices, and our cookies stay with them.
We shouldn't focus too much on how many ads we can pack into a page, but rather try to engage the readers through our fabulous story, and thus gain their trust. The rest is up to them.