Both the technical and practical limitation of minimum dwarves should be four if you're talking about a controlled setup like your embark party. I don't think three would work with divorces since even if you had the two breeding couples from the marriage-babies-divorce-marriage-babies setup (which is obviously quite hard to do), the resulting dwarves should still be listed as siblings, which cannot marry each other. They might be listed as cousins, or maybe even half-sibling if the wiki hasn't been updated (which is quite possible), but it's not practical to try to keep a fort going with an unreliable setup that the three dwarf chain would need, so even if it's theoretically possible we can disregard it as a venue of survival.
Two compatible couples will always allow a fort of arbitrary size to be birthed and sustained, since the only relationship requirement for marriage is that they aren't a parent/grantparent and they aren't siblings.
However, if you're trying to run a fort with as few dwarves as possible and don't have control over who marries who, there's no technical "100% safe" level of population since you could just end up with an entire generation of incompatible dwarves. That said, 30-40 is a pretty safe range to have quite a few options for couples. It is worth noting that the first few generations are a bit riskier, since the ages of your married couples will be very similar at first, resulting in them all dying off in a span of a short few years and just hoping their kids might marry, as opposed to having more sustained die-offs when the ages are spread out a bit more.
Daily interaction should be irrelevant here, since you can almost always force a marriage between compatible dwarves with honeymoon suites. I also use them to ensure dwarves will get another kid cooking right after a birth. Using this method I believe I have nine couples in a fort of 40, and some of that 40 are still kids.