It's a bit tricky. Here's why:
* "Much more" generally implies a greater quantity or degree. For example, "I have much more money than you."
* Negative sentences often express a lack of something. So, saying "I don't have much more money" implies that you don't have a significant amount *in excess* of what you already have.
The problem is that the phrase "much more" can create a contradiction.
Here are some examples to illustrate:
* "I don't have much more time." This can be interpreted in two ways:
* You have *some* time, but not a lot more.
* You have practically no time at all.
Alternatives:
* To avoid ambiguity, it's better to use phrases like:
* "I have very little time left."
* "I don't have much time."
* "I don't have significantly more time."
Exceptions:
* You *can* use "much more" in a negative sentence if you're contrasting two things:
* "This book is not much more interesting than the last one." (Here, you're comparing the interest level of two books, and the negative emphasizes that there's not a significant difference)
In general, it's safer to use "much more" in positive sentences to avoid confusion.