As always, it depends on their role.
If you want a chaff-clearing unit, then 5-6 will get the job done easily.
If you want a unit that will be combat-focused, then no less than 10. Whenever I run my Reiksguard, I choose to have them work in tandem with my IKCs. I usually do this in 1 of 2 ways.
1. I simply put them side by side and charge in together. This can work well by setting up on a flank and sweep around closing the door against your anvil units. In order to do this, they have to be fast, so include the Steel Standard in one unit and the Banner of Swiftness in the other.
2. Put one of each unit on either side and have them meet in the middle. This can prove to be much more tricky with all the chaff and re-directors out there. Plan for those and have something to counter them. This usually works well against armies that can't move very well (Dwarfs) or armies that have "deathstar" formations (Ogres).
As always, much of this depends on placement of terrain.
If you can afford it, Reiksguard and ICKs on one side and Demigryphs on the other side is my favorite. I will put a character with the crown of command in with the ICKs, to make a wall of Stubborn that can act as an anvil for the Demigryphs (hammer) to sandwich the enemy against. If you go with this tactic, try to mirror the size of you ICKs.
