Amazon doesn't release sales figures in the kind of detail that would be required to have a definite answer to the question of how many copies you have to sell daily in order to be in their top 5, but Pubiisher's Weekly took a shot at doing the math.
Their educated guess is about 300 per day. That would vary depending on time of year--in the run-up to Christmas, for instance, you'd have to sell quite a few more because that's the most competitive period.
This is for the "all print books" category. For a while online marketers were offering to sell you a plan to be an Amazon best-seller in a category, like Reference, or a sub-category, like Cookbooks, or Asian Cookbooks. If you go far down enough the line of subtopics, it doesn't take many sales t be number one for a day or part of a day. However, I think just about everybody has caught on that getting all your friends to order your book on one day so that your book is a best-seller in that sometimes very narrow niche for an hour or two isn't all that impressive.
At the moment, the marketing hustlers are offering to tell you how to make a fortune from ebooks. Needless to say, it's wise to regard their hype as highly suspect. Some of them have huge marketing lists in place that they can use to sell many copies of one of their own publications, but that doesn't mean you're going to be able to do it.
The other big hype right now is about webinars. The gurus are offering to teach you how to hold online webinars in order to sell your book or other information product.
Again, they are masters at this themselves and have huge mailing lists and affiliates who push their products, but without that online marketing infrastructure in place, you're not likely to match their results.
Also, often they show you their impressive gross income from selling a particular information product but neglect to mention how much of that gross income went to affiliates (who usually get 50%), how much they spent on building their huge lists, and how many refunds they had to give out. By the time you subtract all those, the numbers may not be so impressive.
