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."

I think you want to say 'what percentage do you want (of...)?' If someone says they got a good score, you can say "what percent?" 78% - Not how much/how many. Does that help?

How many percent? "What do you want for your birthday?" "I want a truck for my birthday." "For what do you want that truck, Sir?" "I want it for my son's birthday" "What do you want for your trip to Paris?" "I want a suitcase." "For what do you want this suitcase?" "I want it for my trip to Paris." I hope this clears it up at least a bit.