Why doesn't Asaj Ventress have a Sith tag?

Right lots to break down

Yes people can follow the sith or look at their artifacts and start to understand the culture. But they are not sith and dont follow the rule of 2, they are not a master or an apprentice.

The sith imperial war
1) not canon, we dont have time to discuss eu
2) It takes place after the rule of 2 is changed. And ventress is alive strictly whilst sith follow the rule of 2

The one sith has these to bare in mind. Next the entire 1 sith illustrates the point that it is 1 sith. And he has followers even if they arent sith unless he steps down. Regardless still goes under the points i made earlier of 1 and 2.

It isnt a mathematical theorem but in practice it is usually followed. Ventress isnt the apprentice at any moment, Dooku was to Palpatine. If palp

The book of Plagueis isnt canon even if the character is. However the book also states that he wished to end the rule of 2 himself, and still had only 2 sith and wished bofr both 2 live forever, just that palpatine wouldnt have more apprentices , which means under the eu darth maul isnt a sith (officially anyway). And the novel states as much, Maul was never meant to learn the ways of the sith, just a killing machine and Plagueis was aware of the whole thing, with Maul none the wiser

Also Sideous continualy followed the rule of 2 with the exception he didnt wish to be replaced, hence why he had vader an an apprentice and why Vader trained the inquisitors yet these didnt count as sith.

Ok, now to explain Tenebrous. Plagueis is the real sith apprentice, but Tenebrous didnt like him and tried to train another to replace him, but he cant just abandon plagueis without a death wish, and Plagueis won. And it makes sense to occasionally have backups if your main apprentice fails, even if they arent an actual apprentice, but again offically there is 2.

You will have to be more specific with vader