I suppose it depends on how active you are for water consumption for example
If i am just driving i can go on 1.5 litres per day
When i was working in the Pilbra in summer 4 to 5 litres per day was not unheard of
When we camp at exmouth as we are swimming we will usually go through 3 litres per day each
So say 3 litres per day per person for a desert trip in winter 6 day crossing 18 litres per person then you may take another 15 litres emergency so say 3 20 litre jerrys should see you through for drinking then 1 more jerry for washing up (you could also use this one to refill a radiator if you had to)
Ps that is purerly drinking you will need extra for washing up etc