As all of writing is a metaphor (the icons on the page represent words, which in turn represent objects, people, feelings and facts) it's arguable that one can't possibly have too many.
That said . . .
The problem usually isn't too many metaphors--it's too many cliched metaphors. ONCE YOU'RE DONE (don't stop writing just for the sake of language; if metaphors keep the flow flowing, flow with it!) go back and figure out:
a) What are you trying to say with this metaphor?
b) Can you say what you're trying to say without it? Can you say it as powerfully without it? Or does the metaphor add something that literal language can't possibly carry?
c) Have you seen someone else use it? If so, can you come up with something better/more apt?
FWIW, an overabundance of similes is much more visible than an overabundance of non-simile metaphors, IMO.
But whatever you do, do -not- stop just to worry at the dry dusty bones of language. Rome may well be burning while you fiddle.
Oh, and don't mix your metaphors in a single thought.
