This question is sort of like the ones asking how much water to carry or how much food to bring. All are very difficult to answer accurately due to the large number of variables.

The answer to your question depends on the ambient air temperature, the temperature of the water when you start, the volume of water your meals require, the wind speed (if any) when you heat the water, the boiling point for water at your altitude, the efficiency of your pot, and I'm sure I'm leaving out one or more.

If I had to give you an answer it would be three. But maybe you'll only need one. It is impossible for anyone here to tell you how much fuel you will consume. Better to have too much than not enough.