I think they are digging deep into the selection of civs, so I expect no fewer than 90 civs by the time this game is complete. If there is indeed an Atomic Age with new civs, I’d bet on a final list well over 100. We can expect more unexpected additions like Nepal, Bulgaria, and Iceland, along with the classic veterans, making the roster truly extensive.
I'm betting the Atomic age does not have new civs in the traditional sense, but something more like a build-your-own civ. But even still, I think between civs which are suggested/begged for, or likely to return, or otherwise would be solid connective tissue, the game will comfortably exceed 80 civs and likely approach or even exceed 100 civs. Just with the first two DLCs and the dataleaked civs, we will be at 43-44. And if we assume, at minimum, that every prior civ returns (I mean, why not? It has been saving them creative burden so far), then adding Franks/HRE, Austria-Hungary, Portugal, England, Scotland, Gaul, Denmark, Sweden, Poland-Lithuania, Venice, Byzantium, Georgia, Morocco, Ethiopia, Nubia, Babylon, Zulu, Scythia, Australia, Canada, Cree/Metis, Mapuche/Guarani, Gran Colombia, Aztecs, Brazil brings us to 68-69 (I excluded Kongo for not linking well to other civs but they could come too). Add in a bunch of candidates which are suggested by what already exists in-game (Wagadu, Ashanti/Hausa, Swahili, Philippines, Edo/Kamakura Japan, Joseon, at least one more Japan/Korea civ, Taino/Arawak, Haiti/Cuba, Burma, Mamluks, Sasanids/Timurids/Seljuks, Halstatt/Gaul, Teutons, Kievan Rus'/Muscovy, maybe Nabataea or Greenland/Inuit or Gutes or Noongar, etc. etc. etc.)...the roster kind of has the potential to balloon. Especially if, as in previous Civ installments, they gradually add mechanics to increase the design space of new civs.