applescript – How to increment a number every time the same automator script runs?

I would like to create an automator script that includes an incrementing number in an email every time it runs.

on run {input, parameters}
    tell application "Mail"
        set myMessage's subject to { INCREMENTING NUMBER HERE }
        set myMessage's content to "body"
    return input
end run

I am assuming the simplest way is to save a number in a text file that this script reads at first and increments at the end. But is there a better way?