Since there is a lag that varies between pending and sold, an exact number isn't easy to come by. We can get close though.
In the past 12 months ending in August, 18,507 properties were sold. If we assume a 30 day lag we would use the past 12 months ending July, 20,928 were pending. This means that 2421 (about 11.5%) didn't close. There is very little difference between the pending sales waiting to close in July and August.
There are many reasons why the percentage is 88.5% instead of 100%. Buyers not getting financing and buyers who did not close because of inspection contingencies were the major reasons. Some buyers and sellers changed their minds and mutually agreed to get out of their contracts.