Since it's not likely to be met. Hell, if there are 6 Maran alive at any one time it's a freakin' miracle, let alone six hero blade imperials (4 elite, 1 warmaster, 1 emperor).
I don't think it's worth mentioning a limit when it's unlikely to be filled (or if it is, for long). It's understandable when you mention leader powers like Piercing Gaze, or Insects. But when it's one sect of the empire and a possible 5-6 slots then it's not likely you're going to get competition in holding the power.
The whole 'being imperial' is a decent drawback though. Those muthers have got a helluva lot of enemies, you just got to learn to gank them more :D
Yhorian