You would use: the # of bins - 1 (sample size) for your degrees of freedom (df) if you knew the (hypothesized) parameters for the mean and standard deviation. An example would be IQ scores withknown parametersof Mu=100 and Sigma=15.

Otherwise, if you are using theestimatesof the parameters for the population mean and standard deviation it would be df= the # of bins - the # of parameter estimates - 1 (sample size).