I really don't think there's any way we're getting 35 leaders at launch.
The number of vanilla civilizations varied from 14 (Civilization I) to 21 (Civilization II) at launch. It is not probable we are getting 35 civilizations in one-go (destroying their business model), so having 35 leaders can only be achievable by having 2 leaders per civilizations, with at least 18 civilizations. This can lead to some problem: some civilizations may not have good record of their leaders, making picking one a problem. We don't want Scheherazade be a leader again.
But if Personas are considered as leaders... Then I can see 100+ leaders! All depends if they are coming back.
I don't see any new leaders' abilities linked to them, though. More a cosmetic option to unlock (and to purchase I guess...). For example, 3 of them could be rewards from achievement (win a game as X, win a deity game as X, do task Y). 1 epic one could be locked behind a GotM challenge, then later purchasable (because FOMO is profitable...).
I am not even sure we are getting 18 civilizations. By logic, we should have more. But we could have less if they plan more flavorful and intricate civilizations, with more mechanics and gameplay experience. The really good impression the players had from the Gallic or the Mayan civilization may lead the designers that way. Civilization VI already put an emphasis to not follow a standard opening, as you need to adapt to your location and your civilization abilities.
Civilization VI even started with less vanilla Civilization than Civilization V. So we may start with less than 18. But really, I bet more on 20, with the 21st being a pre-order bonus. Here a quick history recap:
Civilization I: 14 or 15 civilizations
Included: American, Aztecs, Babylonians, Chinese, Egyptians, English, French, German, Greek, Indian, Mongol, Roman, Russians and either Zulu or Japanese.
Civilization II: 21 civilizations (+6)
Included: Carthaginians, Celts, Persians, Sioux, Spanish and Vikings.
Civilization III: 16 civilizations (-5)
Included: Iroquois (+1)
Removed: Carthaginians, Celts, Mongol, Sioux, Spanish and Vikings (-6)
Civilization IV: 18 civilizations (+2)
Included: Arabian, Inca, Malian, Mongols and Spanish (+5)
Removed: Babylonians, Iroquois and -Zulu (-3)
Civilization V: 18 or 20 civilizations (+2)
Included: ~
Babylonians (pre-order), Iroquois, Ottoman, Siamese and Songhai (+5). Mongols being a free DLC one month after.
Removed: Inca, Malian and Spanish (-3)
Civilization VI: 18 to 19 (-1)
Included: Brazilian, Kongolese, Norwegian, Scythian, Spanish and Sumeria (+6). Aztecs being a pre-order later free to all 90 days after.
Removed: Babylonians, Iroquois, Mongols, Ottoman, Persians, Siamese and Songhai (-7).