Interesting question Ron. As regards stallions I can't think of one who has ND more than once on each side of their pedigree. I presume you mean at least 4 crosses of Northern Dancer? I'd doubt if there is one. You could go onto pedigreequery website and pick a few ND line stallions and then look at the produce page for where they are bred to a ND line mare. I'd feel if you haven't got that you are going to struggle to get 3 crosses let alone 4.
Dansili may be a profitable sire to investigate. He has two crosses of ND, through the Danzig line and the Nijinsky line. With a horse like Passage of Time (by Dansili) you get a third cross as her dam is by Sadler's Wells. She's actually got 4 crosses of Natalma (Northern Dancer's dam). Rail Link may be even more fertile place to go as he already has 3 crosses of ND two on the sire's side and one on the dam's. He's been bred to Night Shift mares, and Night Shift has two crosses of ND. So the offspring of that mating have 5 crosses of ND.
So Dansili. I wonder at what level it becomes too much? A 3X5 stallion with a 3X4 mare to give 4X4X5X6. Because it's the Danehill line you get 5 crosses of Natalma. Will they send him to mares that have multi-crosses of ND? Interesting you learn something every day.
When it's too close (inbreeding). Unfortunately the terminology seems different in horses. I prefer the dog pedigree terminology where there is a distinction between in breeding and line breeding. If there are any faults in the line it's bad news to double up on these as it could take a very long time to be rid of them. In breeding (?within 3 generations) is very dangerous because any faults, however minor, will be magnified. The problems may not even be evident (eg minor heart defect) but inbreeding will magnify these and could produce disastrous results. Line breeding helps to establish desirable features. The only way to improve a particular feature will be to out cross. Ideally the out cross would be to another well line bred animal where the strengths you seek are dominant and there are no serious faults that may be introduced to your line.
