In TPM it seems safe to assume their were two Sith, and they were so undercover the Jedi thought they were extinct.
In RoS we have a giant SafeCo Stadium filled with Sith, a Sith fleet, a Sith language the Republic banned, and secret Sith followers like the bounty hunter who caught Rey’s folks.
Where did these people come from? Was this galaxy spanning religious organization (including secret Sith hood ornaments) spring up in the 40 years between RotS and TFA?
And what do they beleive in? When you ARE A SITH LORD it’s about a struggle for personal power. What does Hooded Sith Enthusiast #35961 in the giant crowd think of all this? Is he happy living in dirt, wearing a hood all the time, waiting for Palpy to make his move?