As others have said, he's nowhere close now. GRRM has sold the better part of 10 million books, Rothfuss is simply not in that league (though if I was told that TNotW had sold more than a million copies so far, I'd believe it), despite being the biggest epic fantasy debut for many years. Maybe a few years from now, if Rothfuss can increase his writing speed and catch up with GRRM. Rothfuss's first book is somewhat more approachable than AGoT and I think could win over a larger fanbase, but the likes of GRRM and Robert Jordan have huge head starts.
Of course, if the TV series is a big hit, that'll get millions more people reading the books. But then TNotW could make a good movie as well (with some condensing you could probably do the three books as three movies), so who knows?
10 million sales is not a relevant number to which series has more fans. even if the 10 million is evenly distributed between the 4 books thats only 2.5 million readers.
Seems like Fiest of Crows sales versus NOTW sales would be the important numbers. Though it sounds like NOTW is way behind.