Try this code
Code:
Sub Macro1()
Dim iWords As Double, sFiller As String, arrFiller() As String, i As Integer
sFiller = "I didn't bother to attempt my homework by myself but I asked someone on the internet to do it for me"
arrFiller = Split(sFiller, " ")
iWords = ActiveDocument.Words.Count
Debug.Print iWords
If iWords < 20 Then
For i = 0 To 20 - iWords
ActiveDocument.Range.InsertAfter " " & arrFiller(i)
Next i
End If
End Sub