While 6-8 glasses or 2-3 litres a day is a recommendation you might hear and is a good rough guide, the exact amount you need to drink varies from person to person. Some people will need more than others and the amount is affected by things like your age, gender, how active you are and how much time you spend inside and outside.
The goal is to drink enough to make sure that during the day you are passing large volumes of very pale straw-coloured pee. If it is dark, you aren’t drinking enough.
Here is a pee chart for you to check to see if you are drinking enough.
You can download the pee chart here to put on the back of the toilet door ;)
Note: If you take B vitamin supplements, that can make your pee bright yellow so be aware that makes the pee colour check a bit tricky!