I have wondered about such things too. One problem we will have in determining it, is that the further along the Path a person has actually gone, the less they will care about whether anyone else knows about it. I read somewhere that one of the characteristics of an arahant, especially, is that they prefer and seek out seclusion.
Now I'm not saying that a stream-enterer is going to be like this too, but I have read the Buddha exhorting stream-enterers to cultivate
seclusion also, to make the effort
to do so.
I'm not trying to pour cold water on your idea - I am also interested. But I think we are 'up against' the issue I mentioned above. This being a Path that leads away
from the World, and thus away
from worldly things such as desire for fame, recognition as a 'great practitioner', etc. We are supposed to give up
all of that kind of stuff. So regarding how many attained there are on the Earth - we will probably never
(When I was about to post this, I saw the post about someone who did
disclose (a few above this one), so I will make a caveat here: it is possible that someone would actually have an experience that was either actual stream-entry or fruit-attainment, and then disclose it with the intention that they might then be able to be of assistance to others.
Now, having the benefit of seeing what happens when persons do this, that isn't something I would do; such claims are almost always met with doubt and scepticism, which means that if they are
actually real, that the sceptics run the risk of offending an advanced disciple! (not that they would take
offence, though, being free from sakkaya-ditthi...
...but anyway...) So being forewarned, I would certainly keep quiet about it. Just to be clear that I am not
disputing the claim of the person mentioned above - could be true for all I know - and I am not
accusing them of pride; no.
I'm just saying, knowing the culture of Theravada, that it is better not
to disclose such things, even if one's intention in doing so was only to help
others (arouse faith that it can
be done, perhaps?).