This is a poetic device. It does not mean that a specific number of sins had been committed, but it indicates that many sins had led to God's judgment.
Here "Damascus" represents the people of the city of Damascus. Alternate translation: "the people of Damascus" (See: [[rc://en/ta/man/jit/figs-metonymy]])
Yahweh uses two negatives here to emphasize that he would punish them. Alternate translation: "I will certainly punish those people" (See: [[rc://en/ta/man/jit/figs-litotes]])