How many themes should a book have before you’re trying to discuss too much?

I know this is subjective and there’s no set answer but I just want to hear opinions. I’m writing a book and my strategy is coming up with a basic plot summary first then asking what themes I could explore there and which ones I actually care about or have enoigu to say about. I think one big problem with writing nowadays is writers with too much on their
minds and I’m generally someone who thinks less is more. But where do you think most books become too specific in their themes or too broad?