How many hours are there in a day?

 Generally ,

1 day = 24 hours

which is said to solar day , it is the amount of time it takes a given spot on the surface directly under the Sun to once again be directly under the Sun.

Actually ,

1 day = 23 hours 56 minutes 4 seconds 179 milliseconds

which is sidereal day, it is the amount of time it takes the Earth to rotate once around its axis .