I picked 8, because that's what it's averaged out as. At the moment I'm buying ebooks in the same way I buy pbooks, lots in one go, then none for a while. Of course, I now have the option of free books, so my to-read pile is growing larger more quickly.
I can see that amount growing, as ebooks are considerably easier to carry home than pbooks
However, as some publishers are still in the dark ages when it comes to ebooks, I will still continue to buy pbooks of novels if I really, really want to read them! Non-fiction is a different matter - sometimes a pbook makes more sense than an ebook (I wouldn't want my reader next to my hob or next to the torch I use for fusing and soldering/brazing, f'r example; and it's no good where colour pictures are an essential part of the book!)
That was a rather waffly answer...