V.W.
Sounds relatively simple.
He drinks 15 oz of breastmilk per day. You replace (pump?) 12 oz. per day. Net loss of 3 oz per day.
15-12= 3 oz per day
So, if you have 19 jars of 16 oz each, using it at the rate of 3 oz. per day, then you would have 5 1/3 days per jar. Which, multiplied times 19 jars is about 101 days. (101 1/3 days actually).
Anybody else want to confirm ?
For Riley: Well, I didn't round anything, and that has a cumulative effect on the numbers. Because as you added another layer of days for each, you run out of days to add to sooner, because of the rounding. Additionally, you neglected to account for the difference in what was consumed (15 oz) vs the bottle size (16 oz). So one bottle does not equal one day. Technically.
In reality, it is probably somewhere in between our answers, b/c the numbers I used do not account for variations in consumption or production that are likely but unpredictable. :))