If a Smart Alert is configured with "On A Single Object" check box enabled, the alert emails can contain multiple objects if they all match the smart alert criteria. (Match the WHO/WHAT/WHERE/WHEN/ORIGIN and the "Send alert when" criteria.)
In addition, the emails will list the events in descending order (oldest on top). In addition, all events that match the criteria will be present in the email, even if they were previously sent in an earlier email.
Furthermore, new events that match the search, but that don't yet meet the Smart Alert criteria will cause an alert email to be sent, if there are still events that match both the search criteria AND the Smart Alert criteria.