It is difficult to know the number of the original users of the bitcoin. We all know that the weakness of the system is anonymous data collected could not be accurate. Perhaps we can only estimate that the number of users without being able to ascertain the truth.
Even estimation is hard since one user can make multiple wallets. By looking at the number of wallets we cannot determine the number of actual users. In this forum it is also hard to compute the number of users since some users here have one or more account. Another thing we cannot get the total number of users verified on the exchangers since some exchangers allow multiple users since they dont ask for valid ids when you registered.