How many twists and turns should be in a great plot in a book series?

The number which the plot demands. Stop thinking of it in terms of twists, and just write your story.

You can't improve a poor plot just by gluing twists on. It will still be a boring book with extra twists.

It depends on how original and engaging the plot it, and how it is written. In some cases, one huge twist at the climax would be the most powerful. In another story, the author might find it necessary to add a small twist every few scenes, to keep the reader interested. In a series of books, an author would probably add a twist or two in each book. But it depends on what they want to write.

I'm not a fiction writer, so forgive me, but I think there is such a thing as too many twists. You may not want to be predictable but your reader needs to know where to look for crumbs if you are going to leave a trail...

I do love plot twists...but even more, I love a twisted premise. ...give me something crazy to accept as a world in which your story takes place. But don't kick all my understandings out from under me to the point where I feel lost or betrayed by your story.

The most powerful plot twists in my mind, are the ones at the VERY end.

if you think the plot is getting too predictable, stick a twist in there. You don't want to overpopulate it with twists and turns bc then the reader just gets confused. I also find it fun to hint to the reader that something's going to happen and then throw a twist in there that makes it seem like its not going to happen and then make the original thing really happen anyway. I guess that's a double twist.