You may use the 1 license for the 1 dev domain and 1 live domain. But keep in mind that you can use our theme license for 1 end product only. Read more – https://themeforest.net/licenses/standard
To activate the theme on new site, you need to deactivate it on old site (via Theme Options).
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.