It does not matter what N you choose. The experimental results are always an approximation of Benford's law. The approximation gets better and better as N becomes larger and larger. The fact that you do not see Benford's law working with N = 19 is not in anyway a flaw in the way the experimental results are set up. It just means N is too small to approximate Benford's law closely.

The larger the value of N is, the more the experiments agree with the Benford's law.