If you title the content control into which you input the number 'NumVal' and a Rich Text content control titled 'NumText' for the words, you could use a content control on exit macro like:
Code:
Private Sub Document_ContentControlOnExit(ByVal CCtrl As ContentControl, Cancel As Boolean)
Dim StrNum As String
With CCtrl
StrNum = Trim(.Range.Text)
If IsNumeric(StrNum) Then
If .Title = "NumVal" Then
With ActiveDocument
.Fields.Add Range:=.SelectContentControlsByTitle("NumText")(1).Range, _
Type:=wdFieldEmpty, Text:="=" & StrNum & " \* CardText", PreserveFormatting:=False
.SelectContentControlsByTitle("NumText")(1).Range.Fields.Unlink
End With
End If
End If
End With
End Sub
no protection required.