The prophecy said that Anakin Skywalker would bring balance to the Force.
Kylo Ren is described as a member of an organization called the Knights of Ren, a mercenary, a powerful Dark Side user, and a fanatical follower of Darth Vader ("I will finish what you started"). But he is not a Sith and is therefore not beholden to the "Rule of Two," which is good because I always thought that was lame.
Which he did in two different ways. Once by creating a balance between the number of jedi and the number of sith, and again in the way the jedi interpretation was expecting when he killed Sidious.
You know who else wasn't beholden to the "Rule of Two?" Darth Krayt, who was also a Sith... and all those Sith who existed before Darth Bane, but fuck it. Let's just go the route of the Star Trek reboot and flip the bird to all the established canon and make everyone swallow what is essentially a fanfiction.