You can use "Set Next Serial Value" script step in onFirstWindowOpen trigger script.
A script can run each time the file is opened. That script can use the Set Next Serial Value step to reset the next value to 1.
But unless there never more than one user opening the file, this is not likely to be workable.
Thank you both for your quick answer. That´s the script step I needed. And yes, only one user. Gracias, de nuevo