You must drink water in order to survive. Generally 8 glasses of water is considered enough to prevent dehydration, and to keep your body well hydrated and to flush toxins.
However, this should not be taken literally. In summer when it’s hot or if you’re physically active, you might need even more water. In the winter, or if you don’t work much, you might need less. Also, if you eat a lot of foods with high water content, such as fruit, you need to drink a little less water.
The best water is filtered tap water, bottled water can be OK as well, especially if you don’t have a filter, or if tap water in your country is not of good quality. That being said, there are chemicals from plastic (bisphenol A and phtalates) bottles that can leak into bottled water, which can cause hormone imbalances.
Tap water, even unfiltered, in some countries is very good, so try to research or test the quality of local water, if it’s very good, there’s no need to buy bottled water. Filter can always come handy though, but sometimes it’s not essential.