Probably nobody knows outside of Disney. But my guess is - not very many. It doesn't take very many blockout tickets before it's more cost effective to upgrade to the next level of AP.
To be honest, I'm not sure how many people buy blockout tickets. I know they do, because they post about it here, but I'm not sure I'd say they are extremely common. They are $50 a pop and so in many cases, it's not much more expensive to just upgrade your AP level to get less blockout days. The only times I can see people really buying them is if it is literally just one day that they will need, or if other friends or family members have planned a trip that's not flexible and they want to join, or if they want to attend a special event (like a ride opening, etc.). Other than that, most of the people that I know that have AP's pretty much avoid their blockout days. I chose a Premium so that I don't have to worry about it.
Ahh, in that case, it will certainly not be a ghost town. Since the 4th is on a Monday, you'll have a larger crowd because people will take long weekends. Disney blocks out AP's because it knows the day will already be crowded, so they don't want AP crowds on top of it. That said, there were reports that the 4th of July was actually surprisingly pleasant last year because of the AP blockout.
yeah, don't plan on that idea. the last two weeks at christmas time are also blocked out and the place is an utter zoo, hitting hard closure. The 4th will be very busy. TLM and SW and July 1st is Opening Day for Goofy's Sky School and also the two new Paradise Pier restaurants. It will be a very very busy weekend.