Not so much numbers, but Yoda balanced Palpatine, Kenobi with Vader, the prophets with the knights etc. Relative power in the force was balanced.
But Plageuis and Palpatine unbalanced it with massacring thousands, because the fall of one person supposedly wouldn't affect the force as a whole, which is why Palpatine's death doesn't make sense as balancing the force when the Galaxy is still being suppressed...
Well, if balance in the Force would be explained better it wouldn't be so confusing. 
I still don't get it, to be honest.