Some of the best books I read have a heap of metaphors, awesomely describing the characters' surroundings or the characters themselves. My creativity appears to be limited in metaphors. I want to avoid clichés, and I want to invent as many of my own metaphors as possible. But how many metaphors should I actually put in my novel?
Not seriously, no.
You really should not be thinking about metaphors. Metaphors are very much an ordinary part of speech. You probably use them all the time without realizing or thinking about it. An non-physical objects, metaphors can't actually be placed in heaps. Heaps of metaphors is a metaphor. But I bet you never thought about creating a metaphor when you wrote that sentence.
The very worst writing seems to result from the attempt to create original metaphors. Metaphors work when they are so apt and natural that the reader never pauses to think, "Hey, that was a metaphor". A forced metaphor has just the opposite effect. It tears the reader out of the story and forces them to confront the prose and puzzle at its obscurity. An good original metaphor is created when the force of a story or a scene creates a moment in which a particular image becomes irresistibly apt. You won't know you have created one until you read it back afterwards.
What you really should be doing is making sure that you have fully imagined the scenes you want to portray, and then describe those scenes in the most concrete and vivid language you can. You will probably end up with a bunch of metaphors without even noticing you are doing it. But if you don't, and the scenes are well imagined and vividly described, it won't matter a scrap whether there are a dozen metaphors or none.
@MarkBaker is absolutely correct. I'll add that metaphors occur in more than just sentence structure.
Even when you write in a way that doesn't include sentence-level metaphors and similes, your readers might find objects in your story that are metaphors for larger themes. Think of Moby Dick's white whale. The Great Gatsby's green light. A River Runs Through It's river.
So I guess my answer is this: Don't worry about it. Just write your story. Use your own voice. Metaphors might appear or might not, and it doesn't matter much either way.