My pharmacist said between 2 and 4, but not more than 4. I think that includes any tea or coffee. I took a flapjack as a sample (yeuck) and she told me to remember to drink some extra water as I would be replacing my shake with a flapjack.
I tend to drink about 3 litres in the day and then have a few cups of coffee. I notice that now if I drink less, I'm thirsty - and even if I don't spread it well across the day, I'm thirsty after a while.
I drink coffee and herbal tea if I'm cold, but I have noticed that coffee seems to make me feel thristier!!