What is a Plot Twist?
A plot twist is a literary device where the author subverts knowledge that a reader already knows or think they have figured out. It’s used to disrupt the flow of the story in a new direction. It’s also used to prevent stories from being predictable.
My two cents
In my writing, I don’t tend to think about plot twists from the start. At times, I don’t think about them at all. It just so happens that as I write, I suddenly think of a twist an include that in the novel. Usually, I think of this as something that just developed from my writing as opposed to purposely knowing I was going to put that plot twist in the story.
I believe that to purposely put in a plot twist, you have to know from the beginning that there is a plot twist. Or at least, had a vague idea of one, otherwise it would seem like the whole idea was shoe-horned in. There’s a fine line between coincidence (and it just happened to turn out that way) and forcing something to happen that shouldn’t. In those instances, the plot twist doesn’t seem natural.
Plot twists aren’t necessary, in my opinion, to make a good story. There might be surprising turn of events but I don’t consider that a plot twist specifically. I feel most plot twist are used in crime/ mystery fiction. Much like romance, there is a certain formula those genre novels follow. IMHO, plot twists are part of that formula.
Whether a novel needs a plot twist or not, is up to the author. Is there such thing as too many plot twists? I would say no but use it sparingly. As a reader, my heart can’t take too many roller coasters so, please be mindful. As an author, I’d use it sparingly when I intentionally use a plot twist.
Dabbling in the Art of Writing 