Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
Range("B14") = Range("B14") + 1
stored within the thisworkbook module within VBA.
This will increment the value in cell B14 (of whichever worksheet is currently active) by one every time you save the file. Closing/reopening will not be counted because you can choose not to save when this happens, and saving changes automatically is a bad idea in case you screw something up and want to discard it...
You won't be able to save as a .xlsx, you'll need to use .xlsm