In thermodynamics entropy S td is an extensive state function. Its derivation by statistical mechanics following Boltzmann and Gibbs with the famous formula S=k B lnW for a micro-canonical ensemble with N particles, k B the Boltzmann constant, and W the number of accessible micro-states is however in general not extensive unless the Stirling approximation given by lnN!-NlnN + N is used. Furthermore, at the thermodynamic limit with the number of particles N→∞ at constant density the Stirling approximation can not be used to show extensivity because lim N→∞ (lnN!-NlnN + N)=∞. Hence, the Boltzmann entropy S as shown here for the ideal gas is neither for a small system with N particles nor at the thermodynamic limit extensive. Thus, if strict extensivity for the entropy is requested the claim of statistical mechanics that the Boltzmann entropy is a microscopic description of its thermodynamic analog is challenged.