>>96304798
It depends on the stage of the war and the legion. At first MkVI was a minority even in the AL/RG despite the flanderization. RG got most of their suits after going back to Terra.

By the mid war, all legions were issuing it in masse to their inductii or to replace damaged suits in addition to MkV. Two exceptions seem to have been:
> EC
Besides inductii, lots of their vets and elites got it, probably out of the novelty?
> TS
Canonically they didn't have inductii or recruited at all, but FW introduced loyalist TS refitted at Terra for the Cthonia campaign. YMMV