You have to loop through the string:
Public Function strCount(str As String, strFind As String) As Integer
Dim intLength As Integer
Dim strHold As String
Dim intFind As Integer
intLength = Len(strFind)
strHold = str
intFind = InStr(strHold, strFind)
Do Until intFind = 0
strHold = Mid(strHold, intFind + intLength)
strCount = strCount + 1
intFind = InStr(strHold, strFind)
Loop
End Function