That depends on how the PDF is being created and served to users.
If you are simply putting an interactive PDF form on a file share or a web site, you could attach script to a form event to generate a unique id but it will not be a sequential number. To create a sequentially unique id you will have to call a server process which maintains the sequential number through a call to a persistent store (a database, XML file, flat file, etcetera).
If you are generating the PDF form using LiveCycle (server solution), the server process used for generating the form could call a process as described above.
Steve