Interesting read... http://www.theappside.com/2012/10/04/app-tracking-service-reveals-54000-download-threshold-for-top-ten-of-app-chart/
That article lacks a bunch of context. Top 10 where? US or worldwide? iPhone? iPad? Article needs a lot more context before it becomes useful. For example, we have a free app in the UK iPhone Top 10 Free chart and the Top 10 iPad Free chart. Combined, they are doing less than 20k downloads per day in that territory.