The actual number of people using bitcoin is not possible to determine.
Even if you track the number of accounts, then also it is not possible because, one user can generate lot of accounts.
Only if you can track the IP of the users then you might get an estimate. But there's no easy way to do it.
That is true, single person could provide multiple accounts in terms of having their transaction, trading and investment. It is really not possible to count the exact people who are currently using bitcoin all we can do is to enjoy the benefit of the support of those people who are continuing supporting bitcoin as a form of cryptocurrency on investing, trading and transactions.