If you want to limit the total number of running instances to one, in Standard Environment, you specify this in your app's app.yaml configuration file, for instance by choosing manual scaling, and setting desired value for the max_instances parameter. Manual scaling uses resident instances that continuously run the specified number of instances irrespective of the load level. More details are to be found on the "How Instances are Managed"
page. Some settings in Automatic scaling may get you the same functionality.