Use ID Generator Field to create a running number in your form design.
Figure 1: ID Generator Field Properties
You 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 the current value of the counter you specified in your app's properties tab.
Figure 2: Environment Variables
Every time the form is submitted, the element will generate the next running number from the environment variable you specified and will place it into the field's value. You may also write the value into a workflow variable.