Ideally for the best health and for enough rest you should aim to get 8 to 10 hours of sleep at night because if you get any less than 8 hours of sleep you may feel drowsy the next morning and may not be as alert as you would be if you had gotten more sleep.
There have been some nights where I only got 6 hours of sleep and I didn't feel very good and wasn't as alert the next day.
I felt like I should go back to sleep and take a nap even though I had to be at work and couldn't and when I only got those 6 hours of sleep instead of my normal 8 or 10 hours of sleep I was not able to concentrate as good the next day at work.
I made more mistakes than I would otherwise if I had gotten my 8 or 10 hours of sleep.
Some people do fine on only 6 hours of sleep and other people like me need a minimum of 8 hours of sleep to feel fully rested.
Not getting enough sleep can lead to heart problems and other health issues as well so you should always try to get as much sleep as possible especially if you need to drive or be at work.