Hola.. te puede quedar de ésta forma:
Cita: Function cont_espacios(ByVal cadena As String) As Integer
Dim i As Integer
Dim num As Integer
For i = 1 To Len(cadena)
If InStr(Mid(cadena, i, 1), " ") <> 0 Then
num = num + 1
End If
Next
cont_espacios = num
End Function
Saludos