My average is about 20 on a clean house up to 40 or so on a POS.
I just reviewed a previous report on a house that took me 33 pages while the previous guy did it in 9. I will have to admit that he did cover most of the issues in pretty good detail but he did not use photos.
I like to use photos, he didn't; personal choice.
The main thing is to cover everything, give your client a good value. I know some guys have 50 page reports, but sometimes lots of that report is boiler plate that may not really apply to the particular house. The "empty" report in TX is 5 pages.
I personally think it needs to be long enough to fully report all issues in an understandable format without trying to recreate "War and Peace."

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002