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.
in fact counting the number of wallet cannot give you the actual number of bitcoin users. there are some people who may be using more than 20 wallets for different purposes and every single usre do not have a single wallet, therefore i think there is no use to estimate the number of bitcoin users just by counting the number of bitcoin wallets.