It seems weird that it is always stated as few. To me it I get the impression it would be under ten people who have. But if there were an entire clan of Uchiha who in battle during Hashirama and Madara's time you think more would have unlocked it as their clan members would be slaughtered in battle next to them. Maybe just one of those things it's best not to even bother trying to question.
You have to remember that the key to unlocking the MS involved having huge love and being powerful. Not all of the clan members
were as loving as Madara, Sasuke, or Itachi. These three thus experienced more pain than the others did at losing someone. Not
to mention, the ability to awaken the MS might also have something to do with the power of the ninja and his or her chakra.
Thus, only prodigies and the greats were able to awaken this dormant potential as many of the Uchiha were simple fodder
that may have lacked the potential altogether. It was mentioned people tried very hard to unlock the powers, so some people
simply didn't have that capability.