If a location is good for one set, its generally good for 2 or 3 sets.
I never throw in more than 3 sets for any canine or cat species, but my trap inventory dictates that guideline for me much more than population or sign at the set location does.
I've tried two different approaches with every fox, coyote, and cat that I've trapped. 1st approach I tried was to flood a location with sets, such as a very large pasture might have got about 10 sets to cover all the possibilities. This approach worked for me when I only had a limited length of time to run sets and I had a limited amount of land to trap.
2nd approach was to acquire access to more land, but limit the # of sets and set locations at each stop. Took more time, but you cover more ground, more populations, and more set locations.
I run more land access now than I used to, and again my own trap inventory pretty much dictates how many and where now for me. I've found that with canines, a couple of sets per location is OK. If its a great location, I may add another pair of sets at the opposite end of the section, if the terrain and sign warrant. Both sets are of different type: one food, other a post or flat, etc. A spooked critter will have me adding 3rd and 4th sets in a completely different type than the first two sets.
For coons, I gang set somewhat. They have closer, smaller territory ranges and the true active season isn't as long before the cold sets in, so I've found it productive to "hammer" them especially on water sets. Same with beaver...and especially on a damage job. Get 'em all and get 'em fast is rule for me, so I gang set everything on those situations. I hammer rat populations like that too. Bank den runs and hut runs get at least 2 or 3 #110s if the run is long enough to keep the caught rats from reaching the other unfired traps. I trap a lot of farm ponds for rats, and if there is enough sign on any of those ponds I generally just fill a bucket with #110s and start walking the banks. In 2 or 3 days when catches fall off, I just move the sets to new runs. Same number of sets, but different locations, in this case.
I don't gang set for mink much, unless there are more than one possible little travel route for them. I use #110s now in blind sets almost exclusively for mink. I may throw a dozen sets in a given stretch of creek, but its mostly due to my limited time to run sets and to cover all the traveling possibilities more than trying to take multiple critters each trap check. I simply cover the best looking travel routes, then as I'm running other traps in the same section I can just take a quick peak at the mink sets on my way by.
I can't recall any time where I just put one trap at any location for any species. My opening comment about a location being good enough for a set then its good enough for 2 or 3 sets was one of the first things I read in Mr. Dobbin's book on land trapping. I've taken that one completely to heart ever since.