Some more discussion here https://www.chess.com/forum/view/general/what-is-the-opening-with-the-most-book-moves-in-chess, including one with 18 book moves https://www.chess.com/openings/Ruy-Lopez-Opening-Marshall-Attack-Modern-Main-Line-Spassky-Variation
What's the most book moves possible in a game?
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Any move that follows conventional theory might be labeled as a "book move" by the opening database in Chess.com. The main lines of the Closed Ruy Lopez (for example), runs through 6.Re1 b5 7.Bb3 d6 8.c3 0-0. And, I guess, 9.h3
After that, I am not sure there is any theory that is so common that Chess.com considers it a "book move", even though some people claim the "main line" extends for another six moves. I personally don't see it.
May 7, 2021
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#4
yeah sometimes chess.com might consider a random move a book move, sometimes even one that has never been played, while other times it doesn't count positions with hundreds of games leading to it. There's actually a way to respond to 1. d4 that isn't a book move, this move is Nh6
There are book lines of 20-25 moves in the Ruy Lopez and the Sicilian.
Jan 14, 2022
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#6
In my "Kids, don't try this at home!" game, the first new move came on move 28. Everything before that was known theory.
blueemu wrote:
In my "Kids, don't try this at home!" game, the first new move came on move 28. Everything before that was known theory.
28 moves theory? 
thats around half of an average game
Jan 14, 2022
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#8
EuweMaxx wrote:
blueemu wrote:
In my "Kids, don't try this at home!" game, the first new move came on move 28. Everything before that was known theory.
28 moves theory? thats around half of an average game
I gave a link to it above, if you're curious.
But for me, I found out
that there's 18 book moves, and that's all. It's called the Ruy López Opening: Marshall Attack, Modern, Main Line, Spassky Variation
What's the longest book move opening? To get an idea, there's a 16 book move opening which you can get from playing the most common moves after e4 e5.