Can you insert number increment in a parent page?

Ah I see. You're probably right. This is how I would approach that problem...

1. Open attached indesign sample file. I've set up the Day numbers in a character style "Day Placeholder".

2. run the below script. It will replace the contents of any text set in that style with an incrementing number, starting at 1. Important note: you can use master pages to place the day number text frames, but they must each be overridden before the script will see them.

- Mark

function main() {

    enumerateTextsInCharacterStyle(app.activeDocument, 'Day Placeholder');


    /**
     * Replaces any text in specified character style
     * with an index, starting at 1.
     * @7111211 m1b
     * @version 2022-08-27
     * @9397041 {Document} doc - an Indesign Document.
     * @9397041 {String} styleName - the characterStyle name.
     */
    function enumerateTextsInCharacterStyle(doc, styleName) {

        var changeStyle = doc.characterStyles.itemByName(styleName);

        if (!changeStyle.isValid) {
            alert('There is no character style named "' + styleName + '".');
            return;
        }

        app.findGrepPreferences = NothingEnum.NOTHING;
        app.findGrepPreferences.appliedCharacterStyle = changeStyle;

        var found = doc.stories.everyItem().paragraphs.everyItem().findGrep();

        for (var i = 0, counter = 1; i < found.length; i++) {
            for (var j = 0; j < found.length; j++) {
                if (
                    found[i][j] != undefined
                    && found[i][j].hasOwnProperty('texts')
                )
                    found[i][j].contents = String(counter++);

            }

        }

    }

} // end main

app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Update Day Numbers');