What is the standard deviation of just one number?
If you have just one number or a million numbers that are exactly the same (such as all are 25), the standard deviation will be zero .
In order to have a standard deviation greater than zero , you must have a sample that contains values that are not the same .
So, at a minimum, you need at sample with at least two values that are not equivalent in order to have a standard deviation greater than zero.
hope that helps