I do find it an interesting topic too - I didn't really have an idea about what would polysaturate me before I found myself in a situation where it was an issue. It's also a topic of discussion because one of my partners said he thought his polysaturation rate was 2.5, two deep emotional and one more casual relationship, and from what I understand he has the two serious ones but is starting to date somebody else, and it just may be another serious one and I'm really keeping an eye out to see if this means downgrading our relationship time/attention wise.
Last year I had one marriage/live in partnership, one long term serious non entangled one date night a week relationship (which I figured couldn't go on for the rest of my life), and one new serious relationship which consisted of one weeknight and one all day long weekend date. I felt like I was at my limit for serious relationships. I was pleasantly full, but was open to having time to the right person for FWB, or a serious relationship if they had daytime hours free (I don't work, all partners work daytimes).
Then I went and met my metamour's partner and next thing I knew I had another deep relationship outside of business hours. There was a bunch of stressful and painful shuffling of priorities, because time wise I just couldn't maintain what I had and have more that was serious, and the new partner is really compatible as a friend as well as a partner, something lacking in my non-entangled relationship that I really craved.
I started seeing my non-entangled partner a bit less recently, and as we don't chat much via IM/email outside of dates, like I figured was likely the case, I am finding I can't maintain the strong emotional ties that way. So I am gauging my polysaturated level at either 3 serious relationship with perhaps one serious friend with very sporadic benefits, or 2 serious relationships with one or two FWB. I also know that I'd rather err on the side of less than more, because when things are stressful with one relationships, it impacts them all, and now I currently have 3 partners who are open to dating /just started dating people and all the incoming stimulus and change and uncertainty is more than I knew I wanted to get myself into, partners with full dance cards is my preference.
My husband and I have been having relationship things that make me feel less close to him for a whole year too, so while I am not 100% sure, I believe I won't feel stretched as thin if that stuff heals.
edit: My 3 time intensive relationship are feeling I am doing a good job being an attentive partner, and I think my non entangled partner thinks I am too, though there are still some awkward times acclimating when we see each other. It's fucking tiring sometimes though, and I am not finding I have as much time for introverted ME as I want. I am glad that going into the future I have a better idea what my limits are, although at this rate I don't think I can afford to even flirt with another human being lest I risk finding a serious relationship when I'm minding my own business.