Most purchased iOS apps with number of purchases?

Nowhere, unfortunately, because the information isn't reported anywhere. Individual publishers sometimes announce their sales and/or number of purchases, but it's uncommon and there's no reporting at all from the majority of the most successful publishers.

There is no official list (available publicly).

Check the blogs of the various developers of popular apps. A few developers release this very private financial information.

The typical (median popularity ranking) or random paid app usually sells around 0 units per day.

According to reports from some analytics aggregators, an app has to get somewhere in the top 10% to 20% in popularity rankings to average $10 or more in sales per day. The apps in the top 2% make most of the money.