hssfworkbook.NumberOfSheets

找到了。
  XSSFWorkbook xssfworkbook = new XSSFWorkbook(fs);
int count=xssfworkbook.Count;

多个工作簿获取可以用
ISheet sheet = xssfworkbook.GetSheetAt(sheetNo);

     
      /// <summary>
        /// 将Excel文件中的数据读出到DataSet中(xlsx)
        /// </summary>
        /// <param name=”file”></param>
        /// <returns></returns>
        public static DataSet ExcelToDSForXLSX(string file)
        {
            DataSet ds = new DataSet();
            //模板中excel有两个 第一个默认规上企业 第二个默认不是规上企业
            using (FileStream fs = new FileStream(file, FileMode.Open, FileAccess.Read))
            {
                XSSFWorkbook xssfworkbook = new XSSFWorkbook(fs);
                for (int count = 0; count < xssfworkbook.NumberOfSheets; count++)
                {
                    DataTable dt = new DataTable();
                    ISheet sheet = xssfworkbook.GetSheetAt(count);
                    //表头
                    IRow header = sheet.GetRow(sheet.FirstRowNum);
                    if (header == null)
                        break;
                    List<int> columns = new List<int>();
                    for (int i = 0; i < header.LastCellNum; i++)
                    {
                        object obj = GetValueTypeForXLSX(header.GetCell(i) as XSSFCell);
                        if (obj == null || obj.ToString() == string.Empty)
                        {
                            dt.Columns.Add(new DataColumn(“Columns” + i.ToString()));
                        }
                        else
                        {
                            dt.Columns.Add(new DataColumn(obj.ToString()));
                        }
                        columns.Add(i);
                    }
                    //数据
                    for (int i = sheet.FirstRowNum + 1; i <= sheet.LastRowNum; i++)
                    {
                        DataRow dr = dt.NewRow();
                        bool hasValue = false;
                        foreach (int j in columns)
                        {
                            dr[j] = GetValueTypeForXLSX(sheet.GetRow(i).GetCell(j) as XSSFCell);
                            if (dr[j] != null && dr[j].ToString() != string.Empty)
                            {
                                hasValue = true;
                            }
                        }
                        if (hasValue)
                        {
                            dt.Rows.Add(dr);
                        }
                    }
                    //datatable添加ds
                    ds.Merge(dt, false, MissingSchemaAction.Add);
                    ds.Tables[count].TableName = count.ToString();
                }
            }
            return ds;
        }