How can I create a Sequence PO number to a PDF fillable form.

It's possible but you'll need to connect the PDF to a server application that supplies the number... and then you'll need to write that application. Is that a possibility for you? 

It's possible if

* you are the ONLY user

* you religiously save the master form every time you change it, separately and as well as saving the filled in form

It's now possible but error prone. You are much better off using a notebook and typing a number in, writing it in the book. Or keeping a spreadsheet.

Let me try to explain what I want in a different way, maybe I didn't know how to express it in a proper way:

I need to add a tracking number to a PDF form. I would like the numbers for each form to be sequential. As an example, I would like to open my form (for the first time) and have the tracking number field show 0001. After I completely fill out the form and save it as a new document, I want that new document to have the number 0001 in the tracking number field, and the next time I open the "master form" I would like the tracking number field to show 0002. Is this possible? If so, how do I do this.

Sounds as if you want something impossible. If it is in the form, what would stop people copying the form (and hence getting two series of PO numbers)? Or not saving (so the same number comes up again).  What you are describing needs a proper database system, not a form. It only seems as if you need forms because that's the part people see - the tip of the iceberg. There are many accounts packages at all levels.