Please explain how that's wrong?

Japanese nouns don't have grammatical number. The closest you can get is adding an affix like -tachi, but even then it's almost exclusively for people and means more like "and company" rather than a true plural.

The noun for "city" is 市 ("ichi"), and it can mean either one city, two cities, or many cities. You can specify how many by adding a number, but the word itself doesn't change.