I'm a believer of the soul mate/twin flame concept because I have met one, yet I am convinced it is not limited to one mate.
Imho, Souls are not inherently 'twined' with another soul (of opposite or same genders) but through a long series or incarnations, they develop karmic bonds with certain kindred souls, and if they meet one or two or three in the same lifetime/incarnation, they naturally attract each other and complete unfinished businesses/tasks collaboratively. The nature of their tasks can be varied ranging from romance, family, an advocacy, or even professional collaboration. However, being familiar with each other for several incarnations (through subconscious memories or soul rays or ashramic/lodge affiliations, or monadic lifewave batchmates), it is easy for them to form romantic relationships and be poly, without much drama.
Before I met my partner, we had several near-miss meetups or contacts ever since childhood through common friends. And up to now, we never run out of common deja vu experiences (inner and outer). The poly outlook is never an issue from the very start with us. That convinced me of the plausibility of the soul mate/twin soul/flames concept.