The most commonly cited guideline, from the National Sleep Foundation and groups like the CDC, is 7-9 hours per night for most adults (18-64), and 7-8 hours for adults 65 and older. That's a range, not a fixed number, because individual needs vary somewhat based on genetics, activity level, and overall health.
The confusing part is that "hours in bed" and "hours of quality sleep" aren't the same thing. Someone might be in bed for 8 hours but only get 6 hours of actual restorative sleep due to interruptions, light sleep, or sleep disorders like apnea. So the raw number matters less than how you actually feel and function.
A more reliable way to judge whether you're getting enough sleep than counting hours is to look at how you feel during the day. If you wake up without an alarm feeling reasonably refreshed, stay alert through the afternoon without needing caffeine to function, and don't feel an overwhelming urge to nap, you're probably getting enough. Chronic daytime fatigue, difficulty concentrating, irritability, or relying on naps and caffeine to get through the day are signs you're running a sleep deficit, even if you're technically in bed for 7+ hours.
It's also worth noting that both too little and too much sleep are associated with worse health outcomes in large population studies, though that correlation is complicated by the fact that illness itself can cause people to sleep more. Consistently sleeping under 6 hours is linked to increased risks for heart disease, weakened immune function, and impaired cognitive performance.
Sleep debt is real too: you can't fully "catch up" by sleeping in on weekends after a week of short nights, though it does help somewhat. Consistency in your sleep schedule matters almost as much as total duration.
If you've been sleeping what seems like enough hours but still feel exhausted most days, that's worth discussing with a doctor, since it could point to a sleep disorder rather than just needing more time in bed.