Well it does not rain much soo they would have to go place to get their water for the day so they would probably try to use the lest amount possible
Well, the average person needs a minimum of 2 litres a day or they have a big chance of dehydration and death.
You might need 5 gallons
Lest just say a lot or you will die
Since water is very scarce the people living at tropical deserts use only very little water for the existence of human life.