You can't really tell how many but you could just estimate. It's all about that like the human population, it's hard to say the actual number. You can't count your hair but you could estimate. It increases everyday for sure, some maybe just use it for business but not always being used.
It's not only difficult to name up the exact number of people that are using Bitcoin, but another difficult point is what people think a Bitcoin user is. Is it someone that is using Bitcoin once or twice a week, is it someone that is using Bitcoin at least once a month... I think the closest point where we can get an answer to that, is when wallet services, exchanges, gambling sites, etc, bundle their power to share user information with each other to come up with a number. It won't be fully accurate, but at least much more accurate than all the guesses we find here.
Exactly, I do not know if someone have mentioned it before but it is a very ambiguous question and as was mentioned by the guy above, if we consider in which period, can be(hours,days, month, and so on) add commonness, etc, so having said that, everybody can estimate the level of use approximately and see if more people are using this or not, one is to see the statistics of the network, for example the volume and quantity of daily transactions in different periods of time.