How many fingers are in a human hand?
A human hand has five fingers.
What about people who have 6 or 7 fingers on one hand? What about people who have less than 5 fingers? Why might someone have more or less fingers on their hand? What are the biological and evolutionary benefits of not having 5 fingers.
An evolutionary benefit of having more than 4 fingers and a thumb would enable a person to play pieces on the piano not possible to  those with 4 fingers. This is explored briefly in the 1997 film, Gattaca. This could be extended to any instrument that requires the use of fingers to manipulate pitch, pluck to activate the instruments resonance or hold a percussive mallet like the xylophone. However it is difficult to imagine an evolutionary advantage to not having a thumb as object manipulation is much more difficult without opposable thumbs. One might postulate the reduction in drag for a swimmer as being one but the reduction in surface area of the propelling hand might negate this advantage. That being said with unpredictable changes in environmental conditions, individuals with less than 4 fingers and a thumb could occur and be selected for over time.