On books made with games of 'human players' the minimum number is believed to be 3, that is to try to avoid the odd game which has been won by 'mistake'... on very large book the number of minimum games tend to rise as must be taken in consideration that some games could be (and you can be sure that are) duplicates.
Duplicate games are the most dangerous think that can occur in an opening book, that is because it alterates the weight of the moves on that line. I saw in this forum people asking if importing more book into one will it make stronger. The answer is obviously not for two main reason:
1) too many line are reduplicates,
2) the manual tuning in all but the last book is lost.
One could object the fact that a strong line, maybe discovered by chance, would not be played anymore thus recopying it several time will assure that it will be played. The problem is that the engine will read the weight at top value altering the valuation of the position which has normally disastrous effect in the result of the game (i.e. it will loose the game).
Book makers and books assemblers.
Here we reaching the sore point of 0 vs. 1. A book maker will start from a database of games and will go trough the book marking moves to play and moves not to play, will also insert manually moves coming from analysis of games, zugzwang moves etc.. This added moves bear the 'zero game played' mark in the opening book. It is here that the value of '0' is necessary to be inserted as value of 'game played' in order to be considered playable. A book assembler in order to use successfully the value 0 must start from a book which has such moves in. As a book assembler is 'just' (it is a very hard work choose the right games for a book in the first place) assembling games without alteration, he should use the value 1 as it has no 'zero games' in the opening book.
Today it seems that some bookmaker has forgotten the danger of duplicate games and seems happy to add them at leisure in their book, so next time that you see a book growing big style from one release and the other you know why...