I need to create a folio field that increments itself everytime you open the acrobat form

Hi, I have a very simple form with a some fields the user should capture.

The first field is folio number, that should be filled automatically when the user opens the form, with the last used folio number plus 1.  The next number available.

I have been reading a lot of documentation but don´t know how should I do this calculation so that if someone opens the form now he gets number 1, the next person gets number 2, and so on.

There is so much information javascript, formcalc, xml, etc.

Can someone tell me which should be the easiest and more efficient way do to this?

Thanks a lot!