How to determine the number of columns in a text box and obtain the width of one column?

Hi @dublove try this:

/**
 * @file Get Column Width of Table.js
 * 
 * Example showing how to get a table from the selection,
 * and then the parent text frame's column width.
 * 
 * @author m1b
 * @discussion https://community.adobe.com/t5/indesign-discussions/how-to-determine-the-number-of-columns-in-a-text-box-and-obtain-the-width-of-one-column/m-p/15550852
 */
function main() {

    var doc = app.activeDocument;
    var table = getParent(doc.selection[0], 'Table');

    if (!table)
        return alert('Please put cursor into a table and try again.');

    var textFrame = table.storyOffset.parentTextFrames[0];
    var columnCount = textFrame.textFramePreferences.textColumnCount;
    var columnGutter = textFrame.textFramePreferences.textColumnGutter;
    var b = textFrame.geometricBounds;
    var frameWidth = b[3] - b[1];
    var inset = textFrame.textFramePreferences.insetSpacing;
    var columnWidth = (frameWidth - inset[1] - inset[3] - (columnGutter * (columnCount - 1))) / columnCount;

    alert('Column width = ' + columnWidth);

};
app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Get Column Width of Table');

/**
 * Return an item with the matching constructor name, if a parent of `item`.
 * @author m1b
 * @version 2025-10-18
 * @param {PageItem|*} item - an Indesign DOM item.
 * @param {String} constructorName - the constructor name to match.
 * @returns {*?}
 */
function getParent(item, constructorName) {

    try {

        while (item.hasOwnProperty('parent')) {

            if (constructorName === item.constructor.name)
                return item;

            item = item.parent;

        }

    } catch (error) { }

};