It can, therefore, be useful to have ideas of schemata that can be used to keep them apart.
I discussed classifying characters by the Cardinal Virtues earlier. Another would be to use the Hippocratic temperaments: sanguine, choleric, phlegmatic, melancholic. Or the classical elements: fire, water, earth, air.
Seven could be the Seven Deadly Sins or the Seven Heavenly Virtues on the good side, or the astrological planets. (C.S. Lewis has a good description of the temperaments associated in The Discarded Image. And could be used for fewer characters if the point is to differentiate them -- and nothing more. But it's a question that needs deep pondering. If the character traits chosen are thematic significant, the tale may need to hit on every important variation, to cover the full theme. (And if they are not thematically significant, they may not pull their own weight.)