Can't able to have more than 15 records in my dropdown (Excel)

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C#

I'm using the below code for making Dropdownlist in excel . in that i can't able to have more than 15 records in my dropdown . Is any properties to denote it ? Please give suggestion

var list = new System.Collections.Generic.List<string>();
        list.Add("Charlie");
        list.Add("Delta");
        list.Add("Echo");
        var flatList = string.Join(",", list.ToArray());

then add this list as dropdown in the particular cell as below

var cell = (Microsoft.Office.Interop.Excel.Range)oSheet.Cells[row, 8];
            cell.Validation.Delete();
            cell.Validation.Add(
               XlDVType.xlValidateList,
               XlDVAlertStyle.xlValidAlertInformation,
               XlFormatConditionOperator.xlBetween,
               flatList,
               Type.Missing);

            cell.Validation.IgnoreBlank = true;
            cell.Validation.InCellDropdown = true;