Вот пример из моей базы
Если немножко переделать суммы на кол-во записей то будет работать
Private Sub Form_AfterUpdate()
Dim wsp As Workspace
Set wsp = DBEngine.Workspaces(0)

Dim us1 As String
Dim us2 As String
Dim nov As String
Dim cnn As ADODB.Connection
Dim rst1 As ADODB.Recordset
Dim rst2 As ADODB.Recordset
Dim VS As Integer
'nalichn = Form_frmKabala.txtNal.Value

'txtVsegoShum.Value = txtNal.Value + txtVsegoChek.Value
Set cnn = CurrentProject.Connection
Set rst1 = New ADODB.Recordset
rst1.ActiveConnection = CurrentProject.Connection
rst1.Properties("jet OLEDB:Inconsistent") = True
rst1.CursorType = adOpenKeyset
rst1.LockType = adLockOptimistic
Set rst2 = New ADODB.Recordset
rst2.ActiveConnection = CurrentProject.Connection
rst2.Properties("jet OLEDB:Inconsistent") = True
rst2.CursorType = adOpenKeyset
rst2.LockType = adLockOptimistic
us1 = ("SELECT * FROM tblKabala WHERE ZakazNom = """ & Form_frmKabala.txtZakazNom.Value & """ ")
us2 = ("SELECT * FROM tblZakaz WHERE ZakazNom = """ & Form_frmKabala.txtZakazNom.Value & """")

rst1.Open us1
rst2.Open us2

VS = 0

Do Until rst1.EOF
nov = VS + rst1!VsegoShum
VS = nov
rst1.MoveNext
Loop


rst2!OplataVsego = VS
rst2!Dolg = rst2!CenaZakaz - VS
rst2.Update

Set rst1 = Nothing
Set rst2 = Nothing

Form_frmZakaz.txtOplataVsego.Requery
Form_frmZakaz.Requery
End Sub