In order to set up default billing series we will go to:
System -> System Configuration
We will open the following window where we will press the Finance option in the left menu and later "Billing Series" as indicated by the red square in the screenshot.
To be able to understand better the options offered by this window, we will start explaining that the numbering would look like this:
We have the options if we want tickets with consecutive numbering, for example (0001-0002-0003 .....)
We can choose the billing series, represented by the letter.We can also decide to add the year or not in the number.