Private WithEvents Items As Outlook.Items

Private Sub Application_Startup()
Dim Ns As Outlook.NameSpace
Set Ns = Application.GetNamespace("MAPI")
Set Items = Ns.GetDefaultFolder(olFolderInbox).Items
End Sub

Private Sub Items_ItemAdd(ByVal Item As Object)
If InStr(1, Item.Subject, "Accepted: ") Then
GoTo SendReply
Else
Exit Sub
End If

SendReply:
Dim oRespond As Outlook.MailItem

Dim RmSize As String
Dim strSubject As String
Dim arrSubject As Variant
Dim arrRmSize As Variant
Dim i As Long

' Set up the array; add more as needed
' one subject and one room size per meeting
arrSubject = Array("Testing this meeting", "Testing the macro", "meeting subject 3", "meeting subject 4", "meeting subject 5")
arrRmSize = Array("2", "3", "6", "4", "50")

' Go through the array and look for a match, then do something
For i = LBound(arrSubject) To UBound(arrSubject)
If InStr(Item.Subject, arrSubject(i)) Then
RmSize = arrRmSize(i)
strSubject = arrSubject(i)

' Start Registry method
' Replace with text code if desired

Dim sAppName As String
Dim sSection As String
Dim sKey As String
Dim lRegValue As Long
Dim iDefault As Integer
sAppName = "Outlook"
sSection = "Meeting Counts"
sKey = strSubject

' The default starting number.
iDefault = 0 ' adjust as needed

' Get stored registry value, if any.
lRegValue = GetSetting(sAppName, sSection, sKey, iDefault)

' If the result is 0, set to default value.
If lRegValue = 0 Then lRegValue = iDefault

' Increment and update number.
SaveSetting sAppName, sSection, sKey, lRegValue + 1

If lRegValue > RmSize Then

' End Registry code

Set oRespond = Application.CreateItem(olMailItem)
With oRespond
.Recipients.Add Item.SenderEmailAddress
.Subject = "Sorry: " & strSubject
.Body = " Sorry, the room is full. You're on the waiting list. " & "You are " & lRegValue - RmSize & " on the waiting list."
' use .display for testing, .send after it is working as desired
'.Display
.Send
End With

Set oRespond = Nothing
End If
End If
Next i
End Sub