most people just using bitcoin for investation
don't know how exactly number of people using bitcoin

Bitcoin is not just an investment platform it has much more wider options than that and if you look at the trading scenario there is a budding option when it comes to trading.
The easiest thing to calculate the actual user data is of the bitcoin wallet. I'm very sure that all transactions are recorded in the wallets.
You really cannot calculate the actual number of people with the data collected from the number of bitcoin wallets used.Everyone uses multiple address for every transaction and so it wont be a definite answer.It is not possible to find out the exact number.