You can run one react native project on a specific iOS simulator device. Open one terminal and run the below command to list down all available simulators :
xcrun simctl list devices
You will get one list like :
iPhone 5s iPhone 6 Plus iPhone 6 iPhone 6s
Now, use the below command to run the project on a specific simulator :
npx react-native run-ios --simulator="iPhone 6"
You can add one new script in default.json with your default simulator :
"ios-default" : "npx react-native run-ios --simulator \"iPhone 6\""
Now, just use npm run ios-default to execute this command.