How many Sith masters obeyed the "Rule of Two"?

Question

We all know that the "Rule of Two" was oft violated and in an audiobook I'm listening to it seems that

1. Even Darth Plagueis' master didn't obey this
2. It says "precious few" Sith Lords did

I am wondering if someone who has read more of the books may be able to actually count how many Sith Lords did abide by it?

Presumably Darth Bane was the first to do so and Darth Sidious did, so that's 2, but how many more? To keep things simple let's count the number of masters who followed the rule (not the apprentices).

If an exact count isn't possible, maybe we can at least narrow it down to a range based on the chronology.

Answer 1

Bane's Order spans about a thousand years and most of the Sith Masters are unknown. We can only consider the few known Sith Masters, so we will underestimate the number of Sith Masters who obeyed the Rule of Two as well as the number who disobeyed it. This means the claim that "precious few" Sith Lords obeyed it can still be true if most of the unknown Sith Lords disobeyed it.

Canon

From Wookieepedia's list of Sith Lords, the following Sith Masters are known in canon:

• Bane: yes
• Bane's unnamed apprentice: yes
• Sith Lord of Gairm: unknown
• Plagueis: unknown. We only know of one apprentice (Sidious) but there could be others we don't know about.
• Sidious: yes. He had multiple apprentices (Maul, Tyranus, and Vader) but he only had one at a time.

Of the known Sith Lords, at least three of them followed the Rule of Two.

Legends

From Wookieepedia's list of Sith Lords from the Order of the Sith Lords (Bane's order), the following Sith Masters are known:

• Bane: yes
• Zannah (Bane's apprentice, only named in Legends): yes, one apprentice (Cognus)
• Cognus: yes, one apprentice (Millennial)
• Millennial: no. Millennial was a heretic who explicitly rejected the Rule of Two.
• Unidentified male humanoid Sith Master: probably. We only know of one apprentice, but we know so little about him that it's possible he didn't strictly obey the Rule of Two.
• Unidentified male Devaronian Sith Master: unknown. We don't know of any of his apprentice(s).
• Vectivus: yes, one apprentice (Gravid)
• Gravid: yes, one apprentice (Gean)
• Gean: yes, only one apprentice but unnamed
• Gean's unnamed apprentice: unknown
• Ramage: unknown
• Darth Tenebrous' unnamed Master: yes
• Tenebrous: no, at least two apprentices (Plagueis and Venamis)
• Plagueis: yes, one apprentice (Sidious)
• Sidious: no. He had a vast number of apprentices, and some at the same time (e.g. Vader and Lumiya).

Note: I don't consider Sith Lords before Bane or after Sidious because those Sith Lords do not belong to Bane's order and thus would not follow the Rule of Two anyway.

I count at least eight Sith Masters who obeyed the Rule of Two, possibly a few more.