You gotta remember that some of the people Itachi killed werent Uchiha at all, since he was killing their spouses... and unless Uchiha is all about incest... well he probably only killed off like 50 max. Its difficult to say since there are basically only 4-5 generations (madara being first) and judging by when they had kids.
Another thing to realize is there werent all that many, Sasuke didnt have any friends in the beginning (before his clan got slaughtered) his brother was basically the closest person to his age, excluding little babies I guess.
Just look at the hyuuga, theyre a lot older than Uchiha and theres very few members (granted that might be because they sacrifice the branch family to protect main whenever neccessary).
All we can really do is guess because theres very little proof to any of it other than the time frame, and small instances of evidence