You may use the ID Generator Field form element 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 variable. You may check on the current running value in the App's properties tab.
Figure 2: Environment variables
Every time the form is submitted, the element would generate the next running number from the Environment Variable and placing it into the field's value. You may write the value into a Workflow Variable.