"I'm worried. I haven't got a single sub plot in my novel and was wondering if that's a bad thing. It is a thriller written in first person (so single viewpoint). The plot is about the hero chasing human traffickers and nothing else. No romance or anything. Should I add some sub plots?"
- Stephen, California
Stephen, novels without a sub plot, just a main plot, are rare - but they do exist, so stick with what you have if you believe that it works.
I believe that stories with a variety of strands to them have greater dimension - as explained in the main article on Subplots. On the other hand, a novel with a sharp, singular focus can be effective, too. Without reading what you have written, it is hard to say.
Also, how many words have you written? Have you produced something novel-sized or is it still a bit thin? Have you had to prune it to bring the book down to an acceptable length (a good thing), or have you had to pad out the main plot to bring it up to size (not a good thing)?
If you believe the novel is the correct length and that it works, trust in it as it stands. You can always add subplots to it later, if the publishers are not interested.
If, on the other hand, you believe in your heart of hearts that something is lacking and/or that the novel is too slim, consider adding a sub plot or two now.
All I can do is give you the "rules" of writing fiction. How you apply them - or choose not to apply them - is down to your own human judgement.
Harvey