What is a Biblical rebuttal of the belief that there is one right person (a soulmate) to be your spouse?
Finding the Right Spouse Not Really Biblical
The history of marriage in the Bible seems to provide little basis for the idea that we must find "the one right person in the entire world." Abraham remarried after Sarah died and, as the saying goes, "had many sons". Abraham's servant chose a bride for Isaac, so Isaac certainly had no opportunity to find the right one for him. Jacob actually chose one and ended up getting the one he wanted and another one he didn't want (or expect).
A problem with the idea of "the one right person in the entire world" is that more often than we would like, tragedy strikes and a spouse dies. That leads to the surviving spouse finding another "one right person in the entire world," who presumably was not the "one right person in the entire world" while the other "one right person in the world" was living.
Finding the Right Kind of Person
There is, however, good reason to seek "the right kind" of spouse. The passage about being "equally yoked" is often quoted to support the idea that believer should not marry unbelievers. It would seem, however, that the verse is broader than marriage and broader than just believer to unbeliever. A large discrepancy in the commitment level of two people to Christ could certainly be problematic. One person who is deeply committed to Christ could probably experience a lot of grief if the spouse prioritizes other things over church and such things as that.
So, it is wise to look for a godly person with integrity and honor who is emotionally healthy, not selfish, who you would be friends with even if you weren't married to them.
Be the Right Person
The Bible actually gives a lot of instructions, not about finding the right spouse, but about being the right spouse. Husbands are commanded to love their wives sacrificially, as Christ modeled in His sacrifice for the church. Wives are urged to respect their husbands.
We are not accountable for the life and actions of others, but we are, indeed, accountable for our own lives and actions. So, we are responsible to become the person God calls us to be.
We should each seek to become a caring, servant-minded, giving, humble, honorable, faithful, godly person of integrity and honor--not just try to find someone like that to marry who then has to put up with us for the rest of their lives.