Veronica-

I do not have a clue as to how many glasses of Grapefruit juice that you can drink in a day.

If  you are looking to lose weight, then you need to read the label and check to see how many calories per specified serving there are.

There are about 3000 calories per pound. What ever you total intake is, calculate the  number of calories that you are getting.

Here is an example. If you need to take in 1600 calories to maintain your weight, then a reduction to 1200 calories would be expected to produce a loss of one pound per week. (400 x 7 = 2800 calories, or about one pound.) If your intake of GFJ is 600 calories, then you get to eat 600 calories  of food.

JayR