Is 8 glasses a day actually the rule?

It's a rough heuristic, not a medical guideline. Actual needs scale with body size, activity, climate, and assigned sex. For most adults, 30–35 ml per kg of body weight per day is a better starting point.

Why does assigned sex change the number?

EFSA sets adequate total water intake at about 2.5 litres a day for men and 2.0 for women, largely reflecting differences in body composition and energy turnover, so the estimate is scaled accordingly.

How much more when it's hot?

In tropical heat and humidity you can lose a litre or more of sweat per hour of exertion. A hot or humid environment adds roughly 750 ml to the daily baseline before any extra for training.

Does coffee count towards it?

Largely yes. Caffeine is a mild diuretic, but at normal intakes the fluid in coffee and tea still counts towards your total. Food contributes too — roughly a fifth of most people’s daily water.