Actually-there are as many personalities and preferences as their are people.
I personally prefer to limit myself to just my two.
My boyfriend is mono, so obviously he prefers to limit himself to one.
Maca has another lady friend-she has several lovers of her own.
It seems to me that if you're talking about a situation where everyone is poly but more than one of them are heterosexual, a foursome would be better than a triad... ;0)
Also-I've seen many situation arise on here where people flat did not want to have a poly-fi (closed) dynamic. they wanted to retain the fluidity of being free to take on a new lover at will.
Ironically-the fear of STD's being totally legitimate, I've encountered MUCH more risk (due to people not even wanting to discuss much less consider using precautions) in the "mono" world than I have in the poly world.
Edited to add:
AND there is always the possibility that a couple could take on a lover who was also the lover to a second couple. So picture Couple-lover-Couple, neither couple is involved with one another, but the lover in the middle is involved in both on a more "part time" basis.....
Lots of options, only the imagination can limit.