As it needed at least 500 games/opening just for the engine to explore few combinations among the "close" moves of the "key" positions, I decided that with 2000 games/opening the quantity of the most effective moves would be quite substantial so as not to have to train the engines again on these openings.
In my opening book, when I add the moves learned at training by the engines, I often add lines until my "out-of-book" 60 plies limit (30 plies for white, 30 plies for black). I try to retain only lines that have at least 100 games (out of 2000 games).