I would estimate at least 50,000 players a day minimum 200k max.
Just because Osu has 7.8 millions users signed up (you round up above 5) doesn't mean that many active. Hell, even if 2 million were active you would see much higher fluctuations in your own rank then we see now. It took 1 year of inactivity for my 80-90k rank to fall to 125~140ish. I'm sure lots of people quit before getting very far. You have to remember that there are a lot of people out there who are just not into quote "anime music" end quote. Out of everyone i know I'm the only one who plays Osu the others quit within the first week. Anyway.
50,000 minimum 200k max.
a decent chunk of this is new users joining while other new users are quitting.
Define active. Do you mean daily players, weekly / weekend players or...