A question I get asked a lot is whether you can have several themes in a novel. The answer (as with a lot of answers in novel writing) is that it depends.
One of the key characteristics of a great novel, in my opinion, is that it has both a unity and a focus to it. For example...
And it is precisely the same thing with themes.
Ideally, your novel should deal with one key issue only. This will be your main theme, the thing your story is ultimately "about." But there can, of course, be all sorts of related sub-themes.
I know this is an article about themes in a novel, but let me begin by talking not about themes and sub-themes, but about plots and sub-plots.
Subplots in a novel only work if they are natural extensions of the main plot. In a story about a bank robbery, for example, the main plot obviously centers around the protagonist's plans to rob the city bank. Including a subplot about his marriage being on the down slope is perfectly acceptable.
What does this have to do with theme? Two things...
First, your primary theme should always remain precisely that. If the novel starts off being about one issue and then drifts off into an exploration of a different issue entirely, you will lose that all-important unity and focus.
(And just because themes are not as tangible as people and places and events, don't think the readers won't notice. They will, even if it is only at a subconscious level - the kind where the book didn't affect them deep down though they cannot precisely say why not.)
The second important thing is that all of the various sub-themes should be closely related to the central theme.
Suppose that a novel is all about grief. The protagonist loses someone who was close to them, and the story's undercurrent then explores how best to overcome the loss of a loved one and keep on living with a broken heart.
Now, the theme of "grief" has all sorts of related issues:
And so on and so forth. All of these issues are slightly different, and yet (to me at least) they are all linked in some way to the novel's overall theme of grief.
Would it be acceptable to use any of the following as sub-themes in a novel about the grieving process...
In my opinion, it would not. (You may disagree, and that is fine - you must always follow your own instincts when dealing with such abstract concepts.)
What you could do is take one of the apparently-unrelated sub-themes and twist it around a bit so that it did relate to grief. Take shyness, for example...
The fact that the protagonist is terribly shy meant that she never expressed her love for her lost one as strongly as she would have liked (and that makes the grief worse).
Furthermore, her shyness now makes it hard for her to find a new significant other in her life (and her inability to move on will make the grief linger).
But shyness by itself is not an obvious complement to the central theme of grief.
Yes, you can have as many as you like. Just ensure that one of the themes remains central. And ensure that all of the supporting themes don't become too prominent.
Of course, I need to throw in my usual disclaimer about theme at this point...
Once you have decided on your primary theme, during the early planning stage, the rest happens at more of a subconscious level, without you having to particularly think about what you are doing.
You will cut out a line of dialogue here, or rearrange a scene there, and somewhere at the back of your mind you will be thinking: No, that snippet of conversation or that piece of action are not contributing to what I want my novel to say; I need to change them to bring it back "on message."
This process is a little like magic.
But it all begins by consciously deciding on a central theme, and one only.
Do that, and implant your chosen theme firmly in your mind, and all of the dozens of secondary themes that will naturally work their way into the story you are telling will support the central theme, not clash with it or threaten to overwhelm it entirely.