Answer:
It isn't the correct dosage, you should get 8 tablets.
Explanation:
First, if the doctors orders 120 mg twice a day, it means that you need 240 mg of medicine. That is calculated as:
120 mg * 2 = 240 mg
Then if each tablet has 30 mg, the number of tablets that you should get is calculated as:

So, 240 mg of medicine are equivalents to 8 tablets.
8 tablets are different of 6 tablets, so the dosage given by the nurse is incorrect and you should get 8 tablets every day.