TMBTM said:

An evil Jedi is not a Sith.

There could be many evil Jedi (Jedi turning bad), but two Sith at a time, according to Yoda (who is in fact George Lucas here...)

But an evil Jedi can become a Sith (or not. He can just stay an evil Jedi).

I know but right from the start in all the merchandising & promotion in 1977 Darth Vader was known as 'Dark Lord of the Sith'......I had a T-shirt with that on it in '77

Sith wasn't mentioned on film till episode 1 in '99, but between '77 and then, if you were into SW you knew a Jedi gone bad was a Sith,...and back then you could probably have a whole host of them