Standard answer… Used to be at least 8, but now some are saying that may be too much, especially for people with problems involving fluid overload. If you're generally healthy & physically active, 8 should still be fine. If your do strenuous work or exercise, especially in hot environments, more than 8 may be needed to replace what you've lost to prevent dehydration.
No set number. You need to consume enough fluids to support your bodies needs.If you pee every 6-8 hours, you have consumed enough to be hydrated. Since some foods (applesauce, vegetables), contain no less than 60% water so a set number of water glasses/day is somewhat of an artificial if not worthless endeavor.Lots of people push this idea, but it has little value.