Re: how much ensure should i drink a day to gain weight?
At 124 you're not underweight. Your BMI is 18.9 and you would need to be less than 18.5 to be underweight. The important question is: Did you lose muscle, fat or both? Drinking Ensure will not automatically bring back lost muscle. If you gain 10 pounds of fat, I'm not sure what the benifit would be to your health, if any. And most of it would likely accumulate on your stomach.
If you have been eating a lot and not gaining weight, I very much doubt that drinking Ensure will be of any help. You need to concentrate on eating a balanced (healthy) diet of natural whole foods. The number of calories you need to maintain 124 lbs. is 2,135. The number of calories you need to maintain 135 lbs. is 2,229. That assumes you're getting light exercise (1-3X/wk.).
There's a limit to how many calories you want to put in any one meal. For example, if you want to eat a meal every 4 hours, you need to have the right number of calories so as to allow yourself to get hungry in time for the next meal. To get your 2,229 calories, you could consume 3 meals of 700 calories each and one snack of 129. See how it works out and you can adjust it if necessary.
Note: In my opinion, Ensure would not be at all healthy for your situation. All the calories come from processed (refined) sugar. That represents empty calories. Then they add vitamins: Formulated vitamins and minerals are too dissimilar to what you would get from natural whole foods. And it would not be calculated for your individual needs. Imagine, if you drink 3 cans, you get 3 times the amount of one can. So you may get too much of one nutrient and not enough of another, depending on your age and individual situation. It may be better than nothing for someone who is suffering from a terminal illness, but it's not for you.