Yes, you're right, Ensure only.The regular Ensure (that is, as opposed to Ensure Plus, which you could also use) is 250 calories per bottle (the US version anyway). I would guess you would need about 6 1/2 bottles per day (1625 calories) if you are sedentary (if you are mainly lounging around the house because you don't feel well, for instance). However, I see from your signature line that you have had two resections. If you have lost a fair amount of small intestine, or have active small intestinal disease at the moment, you could potentially need considerably more calories. Let your stomach guide you. You shouldn't be hungry if you are eating the appropriate amount of formula. It fills you up like regular food does.
If your stomach bothers you when it's empty, you may do best to split the amount of formula into six or more meals per day, so you always have something in there. One caution: because the formula is sweet tasting, sometimes you'll want more even though you aren't really hungry (just like if you eat a cookie than you want another). If you think from the calorie count basis that you have probably had enough, try brushing your teeth and waiting 15 minutes, and then see if you are still hungry or whether it was just the sweetness you were craving.Brushing your teeth and tongue regularly isn't a bad idea anyway, because the formula tends to build on both without the chewing of solid food to rub it away.