You could choose to open and work with an existing workbook. New analysis worksheets will just be appended to existing worksheets in the workbook. If you don't have an existing workbook and want to store all analysis related to your project, you can create a new workbook.
You could also create a new workbook by using the Excel menu to create a workbook. However, make sure that the file you have added has the proper extension. If you want to share your workbook with users who may not have the latest version of Excel, use the Excel 97-2003 file format (*.xls). If you want to use the latest format, make sure to use the Excel format (*.xlsm) so that you can also store macros in this workbook. Macros are used to detect if you have made any changes to the input area so that you can remember to update the output calculations.
You can use a single workbook for all your analysis. It is better to store all the analyses you perform on one project in a single workbook so that all related analyses are stored together. Of course, it is your choice of how you manage your project files. If you have a single workbook, each analysis could be stored on a different worksheet in this workbook.