There really isn’t a set amount of water that one should drink in a day. However is important to note that we lose water in many forms including urine, sweat etc. Therefore you need to replace this fluid loss even of you are not feeling thirsty.
A convention has been to take about 2 litres of water a day. This roughly translates to about 8 250 ml glasses. This is the reason behind the famous 8 glasses/day. However I would suggest rather than counting glasses, take fluid as often as you can.