There is no accurate number that I know of and I would not bother with trying to get one.
I know people have called and gotten wildly different numbers for the same resort. A lot depends on what is called "accessible". Some of the numbers include only the fully wheelchair accessible rooms with roll in showers; some include all the handicapped rooms, whether or not they are fully wheelchair accessible.
I know the people who wrote the Passporter's Walt Disney World for your Special Needs book called a number of times and got different numbers. When I read that part as a peer editor, I advised not putting numbers in. In some cases, the numbers they had gotten were way too small to be the legally required number (for a fairly new resort, so it wasn't like the numbers were low because not many rooms were required to be accessible when it was built). In other cases, the number was unbelievably big - like 25% of the rooms accessible.
No matter how many accessible rooms there are in a particular resort, what matters is whether there is one when the person who needs it wants to stay. The resort could have a large number of accessible rooms, but if they are all taken when you want to stay, you are out of luck. So, to my thinking, knowing they have some (as required by law) is important. Knowing exactly how many they have is not.