http://www.quora.com/How-many-downloads-day-do-you-need-to-be-a-Top-10-Free-iPhone-app
Top 10 free is roughly 30K-120K downloads per day. We had a top five free app getting 120K in one day with an overall rank of #3 before dropping.
http://www.quora.com/How-many-downloads-do-some-of-the-Top-50-free-iPhone-apps-get-per-day
Apps in the top 10 free can range from 50-100K+ downloads a day with the number one free app having the potential to get even more with the correct marketing.