I don't see people taking more than 5 in a unit, though I would not be surprised to see 2 units in a lot of lists. While they are good, even with a 4++ they are still too fragile to take in large numbers. If they get in close combat with a large infantry block they will not be able to break steadfast, they have no banner, and will not have many ranks. And then you want more points for your executioners, hydra/kharybdiss, and just more units for army fortitude.