I've never actually been in more than one relationship at a time. I'm still a poly-virgin.
My relationship with Ben was supposed to be considered Poly, but that really wasn't how it turned out. At the start of that relationship there was someone else I was supposedly seeing, but I only ever saw them one more time before that fizzled out. I kept an eye out for potentials, but very few of them panned out to even a first date.
One of the big guidelines given in many poly relationship guides is a strong recommendation to not get into a poly relationship with someone who hasn't been in a poly relationship before. It's with good intentions that is said, but it creates an exclusive club like the 'you need experience to get a job but you need a job to get experience' catch-22. I have such a hard time getting into one relationship that getting into more than one seems practically impossible. The closet I've come is nibbling around the edges of poly and being friends with people who are poly/open.