I like to believe that there is only one out there but you can have many possibilities because different people can appeal to you in different ways. It's the love that lasts forever that I'd consider the soul mate though.
If you meet him or her there's only one and that's what matters.
The real test is whether that will stay that way,
should you meet someone else
Look at all the people in the world. Look at your own microcosm, and how many roles each person plays. Soul mates come in all shapes and sizes, and play every role in the world.
Only and always--only ONE.