You can achieve your requirement either using Excel formula or simply concatenating the strings before setting it to the cell value. Both approaches are demonstrated below for your reference.
C#
var book = new Workbook();
var cells = book.Worksheets[0].Cells;
cells[“A1”].PutValue(100);
cells[“A2”].Formula = “=CONCATENATE(“Total Number of Days”,” “,A1)”;
cells[“A3”].PutValue("Total Number of Days " + cells[“A1”].Value);
book.CalculateFormula();
book.Save(“D:/temp/output.xlsx”, SaveFormat.Xlsx);
Please feel free to write back in case you have further questions or concerns.