This is not meant as a solution but just a start to give you an idea.
1. You need to create the array of workbook names,
2. You will need to control the starting row in the master where the sum will be entered.

Sub GetClosedSums()
    Application.ScreenUpdating = False
    Dim WorkbookList, _
        workbookToOpen, _
        NumBooks            As Long, _
        RangeSum            As Long, _
        FilePath            As String
        
    FilePath = "C:\Documents and Settings\Benjamin\Desktop\"
    
    WorkbookList = Array("the closed one")        '   <----enter your list here
    NumBooks = UBound(WorkbookList)
    Range("A1").Select
    For Each workbookToOpen In WorkbookList
        Workbooks.Open Filename:=FilePath & workbookToOpen
        RangeSum = WorksheetFunction.Sum(Range("B1:AW1"))
        
        ActiveWindow.Close
        Range("A1").Value = RangeSum
        ActiveCell.Offset(1, 0).Select
    Next workbookToOpen
    Application.ScreenUpdating = True
End Sub