Please take a moment and update your account profile. If you have an updated account profile with basic information on why you are on Air Warriors it will help other people respond to your posts. How do you update your profile you ask?
It's really just trial and error to find the right number per set. You should be completing 9 sets, but the last couple should be difficult. So, if you can't complete 9 sets, reduce the number per set, and if 9 sets is too easy, then increase it.
The typical rule of thumb to start with for your training set is your max effort set divided by 3. So in your case, it would be 6 (18/3). Then from there, you can increase/decrease your effort set as needed according to esday1's reply.
At least that's what I've been told... but it all really comes down to what esday1 said