Jennifer, there is no specific amount of carbs that determines that for everyone. There are others who can explain how it all works. Here is a quick answer, which I hope will help some. :)
The amount of glycogen stored in the liver, and what the pancreas and beta cells are up to is different in each person, for example.
Dr. Wolfgang Lutz recommends 72 grams of carbs per day, to stay out of ketosis.
Dr. Jan Kwasniewski's plan has carbs at plus or minus 10 percent of your ideal Kg weight, which is determined by height in cm. The following is excerpted from this link.
In order to work out the correct daily food intake using this proportion, one has to know how many grams of protein needs to be ingested in a day to satisfy body's requirements. This amount varies from person to person and depends on a "due body weight". Due body weight, in kilograms, is equal to person's height in centimetres less 100 (± 10%). Thus, for a person 160 cm tall, a due body weight is 60 kg ± 6 kg.
A correct amount of protein to be consumed in a day is approx. 1 g per 1 kilogram of a due body weight. Thus, a 60 kg person needs to consume 60 ± 6 g of protein to satisfy body's daily needs. Any excess of protein, above the daily requirement, is converted by the body to fat, provided energy requirements are met by other components.
Having worked out the amount of protein, one can then calculate the amount of other components in a daily menu. Thus, for our typical 60 kg person, the consumption of 60 g of protein has to be accompanied by between 150 to 210 g of fat, and 30 to 50 g of carbohydrate in order to follow the principles of the Optimal Diet.