which of the "selling sth at much more price" or "selling sth at many more prices" is correct? I do not know whether "price" is a countable or uncountable noun because, in the Longman dictionary, it is mentioned that price is countable an uncountable?
That’s a countable use of the noun price (by calling it a price, you’re counting how many prices you mean – in this case just one). And when countable, it can also be used in the plural: They sell at much higher prices elsewhere.
It can also be used uncountably (as the abstract concept of price); for example, in a statement such as: Price is the most important factor when choosing which brand to buy.