QUESTION: How many viewpoint characters can I use? And must I have the bad guy’s point of view?
The point of view character or POV is writing jargon for the person whose head you are inside during a scene in fiction. With the exception of omniscient viewpoint novels, all current genre novels have only one character’s POV at a time.
The number of point-of-view characters you use in a novel depends on genre needs as well as the story you have to tell. If your choice of POVs isn't mandated by the market, you use the number of POVs you need.
In STAR-CROSSED, I used six POVs because my story was so complex, and the novel was big enough at around 130,000 words to allow so many characters. One of the POVs was my villain.
I have also created complex suspense plots with only one or two POVs because the plot was so tightly connected that those POVs were enough. None of those had the antagonist's POV.
If the antagonist doesn't have a POV, the reader will still get a sense of the person because of what he does.
The main characters are also discovering who or what this person is by following the clues of the crime or the situation. As the characters learn about this criminal so does the reader.
If this person's crimes are methodical, this gives the reader a bit of information about him. If he cuts off the victims' fingers with a surgical knife, the reader learns something else about him.
By the time the bad guy is unveiled, the reader should have a very good sense of this character without a POV. At the moment of unveiling, the reader will usually be given the final pieces of this character's emotional puzzle.
Some writers have trouble writing the bad guys because they are concentrating on the good guys and the plot needs of the novel. I always suggest that a writer write a summary of the plot from the point of view of the bad guy starting with the crime, if there was one, and move from that point to the final unveiling.
The bad guy's choices and his story must be as logical for his personality as the plot choices and story of the main characters.