In a day? None - you can go 24 hours without sleeping just fine. On average in a day theories differ, but the lowest commonly accepted answer is four.
However, the better answer would be to say that when you awaken you ought to feel fully alert and bright. If you do not, you are sleeping insufficiently and should look to improving either the quality of sleep of the quantity.
Setting an amount of sleep based on theories of how much a person needs will end badly as every person is different and their requirements change on a daily basis.