There are two ways to change the Settlement limit. You can edit each age and change how many settlements are allowed or you can edit the base and change how much of an effect overpopulation has on happiness, if any.
For each age go to Sid Meier's Civilization VII/Base/modules/age-*/data/ (* each age has its own folder; antiquity, exploration, modern)
Edit the file civilizations-gameeffects.xml Search for the string "SETTLEMENT_CAP" and change the value below.
For the base go to Sid Meier's Civilization VII/Base/modules/base-standard/data
Edit the file civilizations-gameeffects.xml Change the first entry. Should have "SETTLEMENT_CAP" in the string id. Change the Amount and MaxReduction.