Disclaimer: This is not a duplicate of this question. That question deals with subplots being necessary to a story's success, and also loosely how to create them. This question is aimed at knowing when to include subplots, and how many I need. If any.
Let me open by saying that I design my novels extensively before ever writing the first draft. I create my characters and design my plot (amongst other important things), and then write. However, I've recently discovered that I haven't been including subplots in my stories. This leads me to the question below:
How do I know when I need subplots? How do I know which subplots I need to create?
Looking at answers about subplots on this site and others, the general idea of a subplot seems to be that it can show other sides of your characters, making them more real to the reader. It can advance or effect the plot, sending it in different directions. I've even heard one suggestion that it can reinforce the theme on a more personal level.
The point is, you could create a large number of subplots. You could have one for every main, secondary, and tertiary character, a handful to mix up the main plot, and one or two to support the theme. But if you do that, you're going to have way too many subplots.
Your novel has a main plot, a main road it's trying to take. You don't want too many sideroads, because then the main road becomes obsolete. It's no longer the main road. It's just another side-road.
So, to repeat the question, knowing that, how do I decide what to create subplots about? Is there some line I can draw somewhere? Is it at the number of subplots? Perhaps the types of things the plots are about?
How do you know when to include subplots, and how many?