If 18 counters are the whole set, how many are 7/9 of the set?

Any fraction with a numerator equal to the denominator (1/1, 2/2, 3/3 etc) will have the value of one.

Multiplying a value by one will result in the same value.

Multiply 7/9 with a fraction having a value of one that will result in a product with 18 as the denominator. The numerator of that result will be your answer.

Or easier - just multiply 18 (the whole set) by 7/9.