In the U.S., we would say "what percent?" or "what percentage?" The rules of the language say "how many percent," but I never hear people say that.
"Want" is a transitive verb; the sentence must have an object. I would not say "for what do you want?" I would say:
"What do you want it for?"
"What do you want the truck for?"
"What do you want the medicine for?"
You don't always need the full sentence. A conversation at a store might go like this:
Customer: "I want to buy ten sticks of dynamite."
Salesman: "For what?" (Short for: "What do you want that for?")
Customer: "I'm going fishing and I'm too impatient to use a fishing rod."