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.
I definitely agreed on this things, the only thing we can see in the blockchain was the different wallet addresses of the community in the entire world. And bitcoin was pseudonymous, So it will really be difficult to trace it.