Your kidneys can remove 0.8 to 1 litres of water per hour, so theoretically you could drink 20 litres of water in a day. This assumes that you drink at an even pace, though.
There are cases of fatal water intoxication where the victim has drunk seven litres in three hours or less. This sudden influx of water increases the amount of water in the blood, which in turn reduces the concentration of minerals in the blood known as ‘electrolytes’ (these include sodium, potassium and magnesium).
The concentration of minerals becomes less than that inside the body’s cells, and so water moves into the cells by osmosis, to balance the concentration, and the cells swell. In the brain, this swelling is particularly dangerous because the brain is enclosed within the skull, and it cannot expand. So the increased pressure can result in brain damage and even death.