Ideally for the best health you should get between 8 to 10 hours of sleep at night so that you feel fully rested and are alert the next day.
But some people do get by just fine on only 7 hours of sleep so it can vary from person to person.
If you're sleeping 7 hours at night and are feeling fully rested and are not tired and are very alert the next day then 7 hours of sleep is probably enough for you.
But if you're still feeling groggy, tired and not alert the next day then you probably should get 8 or 9 hours of sleep.
Your body will usually let you know if you're getting enough sleep or not and sometimes I can only sleep 7 hours and I'm usually okay but on weeknights when I don't have to be at work I will try to sleep in and aim to get 9 or 10 hours of sleep to make up for the lack of sleep the prior nights.