With Tomb Kings, I'm aiming for a unit of 15! However, this is because when you look at the profile (WS2, S3, 4+ armour save) they really need ranks and numbers to win combats. Admittedly, at 245pts including a command group, it's a bit pricey, but it's not much more than a good-sized infantry unit, and when you hit, the enemy runs.
Of course, TK Heavy Cavalry aren't really knights, but it's the closest we have.
My friend who plays High Elves generally takes a unit of 8 Silver Helms. That does admittedly have something to do with that being all he has, but it's generally enough.
My personal opinion is that it depends upon the requirement. To break ranked up units with a frontal charge, I think you generally need either 2-4 ranks or 2+ attacks per rider. For throwaway, expendable units, as few models as possible. For a general unit 5-6 seems sensible - perhaps 5 and a character to help out?
Then there's Bretonians. I don't play them myself, but the lance formation adds a new dimension to this discussion.