Yes, actually, that's what I want to learn. How many people use bitcoin?
As you say, a person can open more than one wallet. The number of opened wallets does not directly number the person.
This graph also gives the number of online wallet users. We do not know the number of people using a desktop or mobile wallet.
But at least as many people as online wallets are using it. I guess the actual number is 2-3 times that.