By most standards by turn 40 you should have at a minimum two new cities established.
Generally speaking, it’s a good idea to build at least two Settlers before building any districts since expansion is very important early in the game, and this will provide you the means to found those next two important cities. The best rule of thumb is to continue expanding as long as there is space and your top priority is to get your first three cities as soon as you can.
Most experienced players believe you should have about 10 cites by turn 100, meaning you should establish, on average, one city about every 10 turns. The best rule of thumb is to continue expanding as long as there is space. The answer is you can have as many cities as you want without suffering any penalties. But are there any mile markers to go by when building cites, like how many should you have by turn 50? Or turn 100? But the bottom line is that you simply can’t have too many cities in Civ 6. Adding Science producing buildings in them is a must though or you won’t gain much of an advantage.
For example, if pursuing a Science Victory 8 cities is the recommended minimum number. Having a lot of cities will never hurt you although some players might argue that certain kinds of victories encourage you to have a minimum number of cities.
