High King of the Known Worlds
It doesn’t have to be realistic, just fantasy plausible. The teacher obeys the rule of one apprentice because more than one is too much to keep an eye on, and they take an apprentice because somebody has to do the shit jobs. And so long as the Jedi still exist and there’s an enemy that hasn’t killed a Sith for hundreds of years- the “rule” appears to them to be working. No need to break it, because the plan is working, and as far as individual masters are concerned “I’ll be the one to finally end the Jedi, and afterwards I’ll make the rules.” One of the reasons why it’s taken hundreds of years is probably because the masters aren’t passing on all that they know. Much as Palpatine killed his own master before learning the secret of creating life.Except if it's just two guys, and only one of them is the teacher, there's zero incentive to actually pay attention to the "rule." The new teacher gets to do whatever the hell they want. No one's going to punish them for it. And no teacher's going to take on an apprentice with the knowledge or hope of their student one day killing them (or having to kill their student, a valuable asset that they've put a lot of time and effort into making a worthwhile apprentice). It's not a "logical" step, it just looks like what would naturally happen, but there's zero ability to enforce it, and anyone can just decide to take on thirty apprentices, or let their apprentice go off to be their own master with their own apprentice. It just doesn't happen often because of the natural tendency for selfish people who openly channels aggressive mumbo-jumbo to shank each other. It's not like a student-master combo somehow enables easier access to the Dark Side, which would be the one passed-down teaching that actually would move from one student to the next by virtue of having a palpable effect and benefit.
As for the spirit of the rules... those get broken all the time- Palpatine’s Inquisitors, Tyrannus had Asaj Ventress, etc. etc.