adding numbers in a single cell as a running total in that cell ?

That's only possible with VBA. This code needs to code on a sheet, it will watch what is entered into A1 and add to it each time you make a change.

Right-Click on Sheet name
Choose VIEW CODE
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Private Sub Worksheet_Change(ByVal Target As Excel.Range)
Static MyCell As Double

Application.EnableEvents = False
    With Target
         If Not Application.Intersect(Target, [A1]) Is Nothing Then
            
            If Not IsEmpty(.Value) And IsNumeric(.Value) Then
                MyCell = MyCell + .Value
                Else
                MsgBox "The entry in this cell must be numbers only!", vbExclamation, "Wrong Entry"
                .Select
            End If
               .Value = MyCell
        End If
    End With
Application.EnableEvents = True
End Sub

The colored part could be duplicated to set up a second cell to watch...perhaps A2. Just change the MyCell references to MyCell2 in the added section, and insert a Static MyCell2 reference at the top, too.

Play around with it.