My guess is 58 civs in Civ7:
Europe (2 more than Civ6)
North America (2 more than Civ6)
South America (1 more than Civ6)
Mesoamerica/Caribbean/Central America (the same number of civs as Civ6)
East Asian (1 more than Civ6 )
Middle east (1 more than Civ6)
Africa (1 more than Civ6)
Oceania (the same number of civs as Civ6)
Those guesses can be different if they decide to split India, though.
Can we already start to speculate?
Seems like we have.

Here is my conspiracy theories:
American Staples: America, Aztec, Brazil, Inca, Maya. Returnees from previous game: Iroquois as geographically close to Canada's start. New: Argentina, Haiti, Muisca, (replaces Canada, Gran Colombia, and Mapuche), and another western "horse raider" such as Navajo/Apache or Comanche.
Central/South/East Asia Staples: China, Japan, Korea, Mongolia, India, Indonesia. Returnees: Vietnam, and either Siam or Khmer or both if Burma doesn't make it. New: Mughals (separate from India centered more around Afghanistan/Pakistan), Burma/Tibet?
Middle East: Arabia, Babylon, Carthage/Phoenicia, Persia, Ottomans. Returnees: Assyria/Sumer or both.

New: Parthia and Armenia (as Scythia and Georgia replacements)?
Europe Staples: at least England, France, Germany, Greece, Russia, Rome in base game. Duh.

Dutch, Byzantium, Poland, Portugal and Sweden in DLC/expansions. Maybe Spain if not in Vanilla. Returnees: Norway/Denmark for Vikings. New: Ireland (Celts), Some form of Italy, Goths (Classical Era), Franks (if Alexander can have his own civ why not Charlemagne?), Romania if Austria/Hungary don't make it.
Africa Staples: Egypt, Ethiopia (maybe Classical Era inspired over Nubia), Zulu. Returnees: Mali or Songhai, possibly Morocco. New: Berbers if no Morocco, Angola (over Kongo), something more modern instead of Ethiopia like Nigeria or South Africa?
Oceania: Australia, Maori, Hawaii/Tonga/Samoa (as the seafaring Polynesians)
I think I maybe got to about 57 so add in another wildcard somewhere for 58.
Civilizations and nation-states aren't the same thing, though, and most nation-states that have been included are already part of a civilization that's in the game.
This is the clinching point. Australia wasn't included because it's a civilization; Australia was included because Australian dollars.
I mean civilization in game just means playable faction. In the same vein maybe we shouldn't of always had America because America is just a branch of England?
If America can continuously be in the game I have no problem with Australia, Canada, and Brazil though I agree that there does need to eventually be a limit on how many.