They start with 3 settlers on Sid. I think 3500 BC is 10 turns in, and the 2nd and 3rd settlers need a few turns to walk before they found a city. It takes the AI 4 food for a city to grow on Sid. That's at least 2 turns until irrigated... unless it's a flood plain wheat, and I don't think workers work faster. They need 12 shields for a settler. Say a city has a plains cow and the AI irrigates it.
4000 BC 1: Capital city founded, other 2 settlers move.
3950 BC 2: Capital gets 2 shields into the box, worker irrigates.
3900 BC 3: Capitol hits size 2, has 6 shields in the box from using forest.
3850 BC 4: Capital gets 4 more shields in the box for 10 shields.
3800 BC 5: Capital grows to size 3 and produces 1st settler. Irrigation completes on interturn.
3750 BC 6: Capital hits size 2 again, has 4 shields in the box.
3700 BC 7: Capital has 8 shields in the box.
3650 BC 8: Capital hits size 3, 4 more shields for another settler.
3600 BC 9: Capital hits size 2 again, 4 shields in the box.
3550 BC 10: Capital gets 8 shields in the box.
3500 BC 11: Capital hits size 3 and another settler produced.
So, 3 settlers from the capital by 3500 BC. For the other settlers one settler each. So, 5 to 7 settlers seems like the maximum number of settlers by 3500 BC, *given* that turns move in 50 year increments at this time, and accelerated production isn't turned on. We have 10 in the screenshot.