Most of this is from the reading the Darth Bane trilogy, but obviously has been wiped out by Disney.
The "Rule of Two" basically only applies to the most powerful Sith. The rule of two from Darth Bane (and technically from Revan) says: "Two there should be; no more, no less. One to embody power, the other to crave it."
The interpretation is pretty vague, as there are always going to be people studying the ways of the Darkside. Whether intentionally or not. Is it up to the current "Master and Apprentice" to hunt them down? Most likely yes. But in the 30 years without a Master AND Apprentice, it's believable for someone new to have arisen, found a sith holocron etc...
As well, it was not uncommon for both the Master and Apprentice to start looking for candidates instead of just wiping them out.
In the traditional Rule of Two, if the Master is starting to feel his Apprentice is lacking, they might attempt to find another. This could be bad for the Sith as by the time the Apprentice is ready to cut down their master, they might be weakened, thus a weaker apprentice taking over, which could hurt their positions. As well, if the Apprentice find their own apprentice, and they strike down the master... The new Master would weaken the Sith by not being able to overcome the trial themselves.
Much like the old sith Emperor, if I remember correctly, Palpatine/Sidious has been able to extended his life span, and has become so much more powerful in the force, that even someone considered a "Master" would have a very hard time ever trying to defeat him one on one. Which is one of the big reasons why Vader wanted Luke to join him. Together, they might have been able to take Palpatine/Sidious allowing Vader to become the Master finally.
And you notice how Palpatine was goading Luke into striking down Vader and becoming the new Sith Apprentice. palpatine doesn't care if Vader gets struck down, only that someone more powerful would replace him.
It's a tricky situation for sure.
As for Dooku, officially, he is a fallen Jedi who only became Sidious's apprentice AFTER Darth Maul died (unknown if his surviving is currently canon). So there was still only 2 at once. Though the clone wars cartoons gave him a "protegee" Dark jedi, but she never actually became his apprentice and was never given a Darth title.
The big question... is Kylo Ren the Master, or the Apprentice...? (or hell, do they even know about the Rule of 2?)