guess they didn't think about that

Yes.

Palpatine, who literally had Darth Maul, Count Dooku, and Asajj Ventress all working for him once at the same time:

Always two there are, no more no less.
- Grandmaster Yoda, The Phamtom Menace

"Sith"
"No, Not Yet, Perhaps Never"
Jocasta Nu Calling The Grand Inquisitor A Sith, And Him Responding (proof the inquisitors weren't sith)

Ok well there’s always Assaj Ventress, Savage Opress, and Maul living at the same time as Dooku and palpatine

Ventress was given her lightsabers by Dooku, a Sith needs to kill a Jedi and take their lightsaber. Savage and Ventress were also just assassins. Maul gave up his title of Darth, so he problably gave up being a Sith too. Dooku is told to be a Sith by Palpatine, but reqlly he's just a fallen Jedi, as he never became an Acolyte to begin his trials. Palpatine was the only actual Sith

Alright I give up

Ight