Perform a macro on multiple worksheets

Try this

Option Explicit

Sub Autonumber()
    Dim ws As Worksheet
    Dim LastRow As Long

    For Each ws In ThisWorkbook.Worksheets
        With ws
            LastRow = .Range("J65536").End(xlUp).Row
            .Range("A2").Formula = "=if(J2>0,row(A1),"""")"
            .Range("A2").AutoFill Destination:=.Range("A2:A" & LastRow), Type:=xlFillDefault
            .Range("A2", .Range("A2").End(xlDown)).Copy
            .Range("A2", .Range("A2").End(xlDown)).PasteSpecial (xlPasteValues)
        End With
    Next ws
End Sub