You may use the The ID Generator Field form element can be used to generate a running number in your form design.
Figure 1: ID Generator Field properties
Joget would create a new Environment Variable if it doesn't exists. The value of the running value will be stored in the App's environment variableYou can specify the name of an existing environment variable to be used as the number to be incremented everytime a new form is created. If the environment variable that you specify does not exist Joget will automatically create a new one for you. You may check on the current running value of the counter you specified in the your App's properties tab.
Figure 2: Environment variables
Every time the form is submitted, the element would will generate the next running number from the Environment Variable and placing environment variable you specified and will place it into the field's value. You may also write the value into a Workflow Variable.