Where can I see how many free vs paid copies of my book have been purchased by readers?

For any Leanpub book, on the "Sales" page under "Store" you can also see Paid, Free, and Total purchases.

To find the "Sales" page for your book, go here:

...making sure to replace YOUR_BOOK with your book's unique web address.

You can also get information on all purchases of any book on the "Individual Purchases" page under the "Store" menu, e.g.:

There, you can download a CSV file for all your purchases, and you could sort all the free purchases in Excel.

You can find links to both of these pages on your book's Overview page: