15 is ok.

The real issue is getting in your mandatory 2-3 cups of natural fiber - veggies daily. You can still be under with that but most people then add some coffee, eggs, cheese, cream, salad dressings, or sweetner packets which all add more carbs. This is why 20 are allowed.

Atkins used to be lower then 20 carbs but found people were healthier and weightloss was greater at this level.

Take care.