AddTableRows.cs

class AddTableRows : WebEdit
    {
        private string RowsToAdd { get; set; }
        public override void Execute(CommandContext context)
        {
            Assert.ArgumentNotNull((object)context, nameof(context));
            ItemUri queryString = ItemUri.ParseQueryString();
            if (queryString != (ItemUri)null)
            {
                Item obj = Database.GetItem(queryString);
                if (obj != null && !Sitecore.Web.WebEditUtil.CanDesignItem(obj))
                {
                    SheerResponse.Alert("The action cannot be executed because of security restrictions.");
                    return;
                }
            }
            NameValueCollection parameters = new NameValueCollection();
            string id = context.Parameters["id"];
            parameters["id"] = id;
            if (string.IsNullOrEmpty(id)) //using button via exp editor so validate rendering
            {
                string parameter1 = context.Parameters["referenceId"];
                Assert.IsNotNullOrEmpty(parameter1, "uniqueid must not be empty");
                parameters["uniqueId"] = parameter1;
                string parameter2 = context.Parameters["renderingId"];
                Assert.IsNotNullOrEmpty(parameter2, "rendering item id must not be empty");
                parameters["renderingItemId"] = parameter2;
            }
            else //using button via content editor so validate item clicked
            {
                Item tableItem = Client.ContentDatabase.GetItem(id);
                Assert.IsTrue(tableItem.TemplateID == ID.Parse("{408F8C86-59EC-475E-8BF5-5C24D3A3D1F4}"), "This item is not a Table so Rows can not be inserted.");
            }

            Context.ClientPage.Start((object)this, "Run", parameters);
        }

        [UsedImplicitly]
        protected void Run(ClientPipelineArgs args)
        {
            Assert.ArgumentNotNull((object)args, nameof(args));

            if (!args.IsPostBack)
            {
                //ask how many rows (maximum of 50 at a time)
                SheerResponse.Input("Enter the number of rows you would like (max 50):", "1", @"\b(0*([1-9]|[1-4][0-9]|50))\b",  Translate.Text("'$Input' is not a valid number."), 50);
                args.WaitForPostBack();
            }
            else if (args.HasResult) //user has entered rows or columns
            {
                if (string.IsNullOrEmpty(RowsToAdd)) //rows entered
                {
                    //set default cols to add based on prev rows - if there are some
                    var colsToAdd = "1";
                    Item tableItem = Client.ContentDatabase.GetItem(args.Parameters["Id"]);
                    if (tableItem!=null && tableItem.HasChildren)
                    {
                        colsToAdd = tableItem?.Children[0]?.Children.Count.ToString();
                    }
                    //ask how many columns (maximum of 30 at a time)
                    RowsToAdd = args.Result;
                    SheerResponse.Input("Enter the number of columns you would like (max 30):", colsToAdd, @"\b(0*([1-9]|[12][0-9]|30))\b", Translate.Text("'$Input' is not a valid number."), 30);
                    args.WaitForPostBack();
                }
                else
                {
                    Item tableItem = Client.ContentDatabase.GetItem(args.Parameters["Id"]);
                    AddRows(RowsToAdd, args.Result, tableItem);
                    RowsToAdd = string.Empty;
                }
               
            }
        }

        private static void AddRows(string rows, string columns, Item dataSourceItem)
        {
            int rowCount = dataSourceItem.Children.Count + 1;
            int rowsToAdd = int.Parse(rows);
            int columnsToAdd = int.Parse(columns);
            var rowTemplateId = new TemplateID(new ID("{36257DFF-0E30-472C-B269-636569FAAF25}"));
            var cellTemplateId = new TemplateID(new ID("{2FBBDAF7-6B2C-4A7E-B1A3-E270273EB603}"));

            using (new Sitecore.SecurityModel.SecurityDisabler()){
                //add rows (depending on how many rows we have already we may need to start this count from more than 1)
                for (int i = 0; i < rowsToAdd; i++)
                {
                    var row = dataSourceItem.Add($"Row {rowCount++}", rowTemplateId);
                    //set sort order
                    row.Editing.BeginEdit();
                    row.Fields[Sitecore.FieldIDs.Sortorder].Value = (rowCount-1).ToString();
                    row.Editing.EndEdit();
                    //add cells to rows
                    for (int c = 1; c <= columnsToAdd; c++)
                    {
                        var cell = row.Add($"Cell {c}", cellTemplateId);
                        //set sort order
                        cell.Editing.BeginEdit();
                        cell.Fields[Sitecore.FieldIDs.Sortorder].Value = c.ToString();
                        cell.Editing.EndEdit();
                    }
                }
            }
        }
    }