finding number of "active" players might be impossible but you can get a rough estimate of the number of players currently not so innactive as to have been de-ranked. So I went on to Osu and sorted by rank and found the lowest ranked online player I could.
Found one that listed as 0PP and was rank #3,391,360. Now this is very likely not the actual lowest ranked player as it was rounding this person down to 0PP and there are probably plenty of other people who have an even smaller fractional amount of PP.
The reason why this number is so much less than the 11Million users is for a few reasons.
1. The game de-ranks players that have not played a specific gamemode in a long enough time. Since Osu is 10 years old, there are tons of innactive players yet the user account number constantly increases.
2. There are many users who join Osu to play other gamemodes like Taiko, Mania, or CTB and never even touch Osu Standard.
3. People that created an account and never passed any beatmaps