Only one, plus scheduled reminders.
If an individual is named as a Client Referee by multiple firms, and/or for multiple practice areas, then the one email from us will include multiple links (enabling them to comment on all those firms, and all those practice areas).. This may reduce the chances of getting a response but we do it to avoid client-fatigue (ie clients being sent too many emails), and it is why we always advise firms to use a wide range of Client Referees.