There is at least one other, when his device isn’t broken.
Jack was a member of a human Time Agency. So, who knows how many there are…will be…have been…
… Stupid time travel.
One other Agent has appeared - John Hart (who makes Jack seem absolutely puritanical, sexually…and in most other ways, actually).
I haven’t seen it yet myself. That’s why I have to plug my ears and say “la la la la la” when I read this thread.