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's right, its not easy to do it. Only the wallet addresses can be traceable in the blockchain history records. But as everyday pass by a thousands of users are getting involved into bitcoin. So its difficult to determine it.
Even with the wallet addresses generated it's not possible to determine the number of people using bitcoin. Blockchain has got more than seven million wallets. Likewise other wallet providers too have that. I believe bitcoin might have 3-6% of users worldwide.