It's true, monogamy and polyamory have a great deal in common. Both are made of the basic stuff that makes up relationships.
They say, don't start a poly relationship if one of the members has a psychological problem, don't start one if your existing marriage is out of sorts, don't fall in love with someone new and then ask for poly in order to hook up with that new someone, things like that, and yet, the V/triad I'm in "broke" a bunch of those "rules" when we came together. Mind you, we a few rough years together in the beginning while trying to figure out how to make it all work. But we basically did figure out how to make it work in the end, and the solutions have been uniquely ours.
The crossover of relationship issues between monogamy and polyamory means, for example, that "The Five Love Languages" (by Gary Chapman) is a great book for poly families to read, even though it was written by a conservative minister who can be seen to consider monogamy the "one and only healthy relationship model." The Love Languages idea is so simple and universal that it naturally extends beyond the bounds of monogamy, and helps many polyamorists.
No doubt the biggest part of the crossover realm is the seemingly limitless potential that good communication offers in the way of establishing emotional intimacy, and just plain old comfortable dynamics of learning how to work together like a well-oiled machine. Communication is a never-perfected skill. One can always learn how to become a better talker, and a better listener. The principles of honesty and kindness go hand in hand with the skills of good communication, and all of these things can work wonders for monogamous relationships, just as much as they can for poly relationships.
So yes, the numbers are often an incidental part of the equation.