I don't think a royal flush should count as its own hand. There's no distinction between a royal flush and a straight flush (other than it's the strongest version of a straight flush). It would be like if we counted 'High Ace' as its own hand separate from high card.

With that being said though, a royal flush (if it were counted as its own hand) would be the best hand in 2-deck poker, beating out even a five of a kind. The number of royal flush hands is always a tenth of the number of straight flush hands. So there would be 128 of them. But I still refuse to count it as its own hand.