I don't know how many Finders Keepers proofs were done. I think #272 is about the largest number I've seen for this one.
There is no "average number" of proofs done. It varies widely between the publisher and the book itself. There were supposedly 200 copies of the UK Revival proof done while there were supposed to be 150 copies of the UK Doctor Sleep. The US publisher (Scribner) usually does not disclose the number done. According to my notes, about 10,000 copies of the US Rose Madder proof were done.
I should add that the Scribner proof of Cell is extremely scarce. At one time it was said that they did not even produce one, although I had one at the time. I know of about 4-5 in collectors' hands currently.
John