research will lead to more guesses as exchanges and other popular services won't release any details about how many unique users they have. surveys online and in the streets won't do anything either as people can simply make up everything. especially when they get a small reward for completing the survey.
That's not the main problem, i think there's no major benefits of finding out actual number of bitcoin in this world
Maybe would be better if there's research about what people know about bitcoin and why they don't want to use bitcoin.
no benefits indeed, but more for statistical purposes for what it's worth. actually, back in 2014 there has been a major news channel having done some research by asking people in the streets whether or not they know what bitcoin is. the people that have heard about it compared bitcoin with the tulip mania back then. they also said to not use bitcoin because they can't hold it in their hands like they do with cash money. that's ignorance from the highest levels.