I run mine 2x6. It gives a little more hitting power, and you get an extra couple of wounds for the purposes of denying the enemy rank bonus and that means the enemy unit will almost certainly be broken in combat. With standard knights I haven't found they improve with any greater numbers.
I do run greatweapon knights in larger units, as I use these guys to grind an enemy unit down over a few turns. I've even stretched these guys out to 30 for the horde bonus and they've done really well when they've been given the right target.
Malleus wrote:... and Skaven will steadfastly lul at you for 6 turns.
The mobility of those knights mean you're very likely to get into the flanks of the enemy, and with two ranks of five that's enough to deny the rank bonus to leadership... which makes testing
LD, even with steadfast, a very unreliable prospect. With General &
BSB nearby, without the rank bonus a unit has about an 80% chance of holding, and that drops every turn. Move the general or
BSB away from that and the chance of failure climbs very quickly.
Also, steam tanks are completely wretched and nobody should ever take one.
That's just simply, objectively not true. They're no longer the T10, 10 wound brick of points denial they used to be, but they're also capable of continuing to deliver the hurt after they've suffered a few wounds now.
Knights need large blocks of infantry to clog people up while the knights deliver the KO, and can't really operate independently in the world of steadfast.
That's certainly the most common way of running them, but it isn't the only way viable way. Lists full of Empire knights with 1+ saves are looking pretty solid now, giving up very few points over a game, and with the mobility to gang up on smaller enemy units.