It's 27 volumes or 15 deluxe sized ones.
It's big, but when I think of long manga, it doesn't seem that long.
Yep it's nothing compared to something like the length something like Naruto (72 volumes in total), One Piece (86 and is ongoing), or Dragon Ball / DBZ (42 volumes in total)
In the scope of things Fist of the North Star at 27 volumes, saying it is very long doesn't hold as a reasonable excuse for the sake of translation.
I don't think the age or length of the series had anything to with it's lack of popularity in the West. I think it's more so of a marketing thing. Dragon Ball came out at the same time as HnK being early 1980's. For western marketing Dragon Ball was just pushed much more. If HnK got the same push it's popularity could easily be on par.