VBA To Create auto Serial number based on row of newly created sheet data

[earthworm]

I Have a workbook that contains data . I created a macro that copy the selected data and paste the same in new workbook . and create serial number as per data pasted on newly created workbook . I tried below macro . But what this is doing is its creating serial number based on previous work book and pasting in newly created workbook . Please help me what am i doing wrong. I even tired to select active workbook select before below code but still there is same issue .

'Auto fill Serial Number
    
    If WorksheetFunction.CountA(Columns("B:B")) < 2 Then
    Range("A2").Value = 1
    Else
    
    'To Add serial number if there is more data in column
    
    Dim y As String
    y = Sheet1.UsedRange.Rows.Count
    Range("A2").Value = 1
    Cells(2, 1).AutoFill Destination:=Range("A2:A" & y + 2), Type:=xlFillSeries