how can i get a grand total of submissions

It would depend on what type of total you are looking for.  If you are trying to get a total of values entered across all submissions, exporting to Excel format and totaling up the values in the sheet would likely be easiest.  If you want to view how many times a specific option was selected, a visual report would likely be your best bet:

https://www.jotform.com/help/187-How-to-Create-a-Visual-Report-with-Your-Form-Submissions