Ooh, so you're saying just because it's OCC that # doesn't change? Well, then we use the 1st thing I said about score.
I've just checked in the test game, and the number of cities only updates on the scoreboard when one of our units sees the AI city.
So we need to use indirect means to tell
if Sitting Bull made a 3rd city.
Here is the
Turn 56 to T65 Demographics.
T65 Graphs thanks to

Sitting Bull so annoying, even his graph lines are stealthy and hard to see!
World Buddhism Influence
T55 - 18%
T56 - 17%
T57 - 17%
T58 - 16%
T59 - 17%
T60 - 16%
T61 - 15%
T62 - 15%
T63 - 15%
T64 - 11%
T65 - 11%
World Hinduism Influence
T55 - 6%
T56 - 7%
T57 - 7%
T58 - 7%
T59 - 7%
T60 - 8%
T61 - 8%
T62 - 8%
T63 - 6%
T64 - 6%
T65 - 6%
T65 Sitting Bull's capital Cahokia is Size 2 and has just a Castle and a Palace.
T65 Poverty Point is Size 3 and has a Granary in the center of the city.
Here is the espionage production data for each city.
Using the formula:

in AI City build = (Sabotage Production / City Revolt) * (650/6)
Cahokia (Capital) 
T55 - 51
T56 - 57
T57 - 63
T58 - 69
T59 - Building something new
T60 - 15
T61 - 18
T62 - Building something new
T63 - 4
T64 - 3

T65 - 9
Poverty Point 
T55 - 3
T56 - 4
T57 - 5
T58 - 6
T59 - 8
T60 - 10
T61 - 12
T62 - 14
T63 - 16
T64 - 18
T65 - 20
Cost for all buildings and units on Emperor for the AI is 85% (in the Ancient Era) unless it is a Settler.
Settler = 95

Worker/Lighthouse/Granary = 51

Barracks = 43

Workboat = 25

Archer = 21
Wonders are full price.
===============================
Sitting Bull Score
T55 - 136
T56 - 136
T57 - 138
T58 - 147
T59 - 150
T60 - 150
T61 - 159
T62 - 159
T63 - 168
T64 - 177
T65 - 183
===============================
Cahokia Population
T55 - 2
T56 - 2
T57 - 2
T58 - 2
T59 - 1
T60 - 1
T61 - 1
T62 - 1
T63 - 1
T64 - 2
T65 - 2
Poverty Point Population
T55 - 1
T56 - 1
T57 - 1
T58 - 2
T59 - 2
T60 - 2
T61 - 3
T62 - 3
T63 - 3
T64 - 3
T65 - 3
Cahokia 1whip settler? Pop2>1 now?
Yes, on Turn 59 Sitting Bull had a new Settler for sure.
Well, then we use the 1st thing I said about score.
1 extra empire population is instantly +6 points on the scoreboard I think, but it might depend on how much food is on the map.
Turn 63 has a suspicious score increase for Sitting Bull I'd say.
He settled his 3rd city on Turn 63?
Or maybe Turn 65?
Or he got more land 20 turns ago or a new tech? (I'm not really sure how to read score)
With such low population and production, I don't think Sitting Bull can easily make another 95
Settler if he already used his T59 Settler.