why 10 glasses of water per day?

Hi, morpho99. 10 glasses is a general amount – each person’s need is based on their size and degree of losses (i.e. from heat, physical activity) and any medical conditions. Water is needed for temperature balance in the body, to flush out toxins (including everyday byproducts of normal metabolism), and to keep cell membranes and blood flow functioning. Not drinking enough causes dehydration, which in significant cases, leads to shutting down of the functions listed above, and ultimately death. In milder cases, it makes skin crack and wrinkle, contributes to constipation, and ironically contributes to overeating in some cases. In diabetics, it makes high blood sugar harder to control. Some medications are also less effective, i.e. some pain killers.
If you would like a closer estimate of your personal needs, you are welcome to list some relevant details here, and I will run some calculations. These details include your age, gender, height, weight, physical activity level, tendency to sweat, fiber intake, and any medical conditions, including diabetes, kidney, heart/circulatory, liver, and digestive, and medications you are on.
Drinking with meals can be a very individual thing. Ultimately, you need to listen to your body. If you feel your digestion is not what it should be, try not drinking with meals – if this helps, if could be a sign that your enzymatic activity should not be diluted; if it does not, and you feel better drinking with meals, then continue. Remember that liquid is also a lubricant for digestion. As for timing of the last glass of water, there is no limit there.

Happy to help!