I think finding someone who you have red hot passion for, a joint commitment to growing together and a great friendship / sense of sharing all combined together is rare.
Meeting that right person, at th right time in both your lives for you to offer the commitment and appreciation of the value of it without outside forces influencing is very low actually.
So I think, yes, thre is a a thing as "the one", as in, the one person who ignites all of that in you at the same time -but I think there could be an infinite number of those perfect people floating around or there could be none.
Finding "one" of them at the right time, is not something that happens to everyone in this life -so I always consider that lucky enough to warrant all the fandango.
People who say we choose "the one" through commitment to a life with them aren't being real. I committed to a man once, who I loved very deeply, and felt very much love for. We made a life and home together but in the end I was missing that je ne sais quoi that you can't create, choose or control in any way.
That magic something is something I have felt twice in my life.
Once when very young, when we were too young to make a proper life of it and again when I was older and he wasn't ready to commit because he was younger than me.
I may or may not find that again with someone, or I might need to settle for someone I feel another kind of love for.
Life isn't easy, but yes sometimes when all the stars and planets collide at the right place and time for the fireworks to happen with someone when you are both on the same page...then that...that is "the one".
Not to say the only one ever....but even having one like that is a pretty wonderful little miracle.