A stack isn't necessarily limited to eight units; that's just the point at which
the bonus maxes out (and so increasing the stack beyond that point runs into diminishing returns, and perhaps other limits that haven't been described yet).
That said, it's possible that veiling can only cover so many units. (I note that if it can't cover at least 22 units, it wouldn't have been an option for hiding the wounded dwagons and warlords even if Parson had known about it and had been able to arrange for the Foolamancer to be available on the scene).