<PERSON>'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:
* <PERSON>: yes
* <PERSON>'s unnamed apprentice: yes
* Sith Lord of Gairm: unknown
* <PERSON>: unknown. We only know of one apprentice (<PERSON>) but there could be others we don't know about.
* <PERSON>: yes. He had multiple apprentices (<PERSON>, <PERSON>, and <PERSON>) 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 (<PERSON>'s order), the following Sith Masters are known:
* <PERSON>: yes
* <PERSON> (<PERSON>'s apprentice, only named in Legends): yes, one apprentice (<PERSON>)
* <PERSON>: yes, one apprentice (<PERSON>)
* <PERSON>: no. 605
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).
* <PERSON>: yes, one apprentice (<PERSON>)
* <PERSON>: yes, one apprentice (<PERSON>)
* <PERSON>: yes, only one apprentice but unnamed
* <PERSON>'s unnamed apprentice: unknown
* <PERSON>: unknown
* Darth <PERSON>' unnamed Master: yes
* <PERSON>: no, at least two apprentices (<PERSON> and <PERSON>)
* <PERSON>: yes, one apprentice (<PERSON>)
* <PERSON>: no. He had a vast number of apprentices, and some at the same time (e.g. <PERSON> and <PERSON>).
Note: I don't consider Sith Lords before <PERSON> or after <PERSON> because those Sith Lords do not belong to <PERSON>'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. 605