The old "8 glasses a day" rule is a rough rule of thumb, not a precise target. The more useful guidance from health authorities is around 11.5 cups (about 2.7 liters) of total fluid per day for women and 15.5 cups (about 3.7 liters) for men, and that includes water from food, not just what you drink. Roughly 20% of daily fluid intake typically comes from food.
Your actual need depends on a lot of factors: activity level, climate, body size, pregnancy or breastfeeding, and certain health conditions all push the number up or down. Someone exercising heavily in hot weather needs meaningfully more than someone sitting in an air-conditioned office.
A simpler practical check than counting ounces is to watch your urine color and thirst. Pale yellow urine and rarely feeling thirsty generally mean you're adequately hydrated. Dark yellow urine, persistent thirst, fatigue, or headaches can be signs you need more fluids.
It's also possible to overdo it. Drinking excessive amounts of water in a short period can dilute sodium levels in the blood (hyponatremia), which is dangerous, though this is rare and mostly seen in endurance athletes or extreme cases. For most people going about a normal day, that's not a real risk.
Bottom line: there's no single magic number that applies to everyone. Drink enough that you're rarely thirsty and your urine stays light colored, and adjust upward for heat, exercise, or illness.