There's one zener in it. That sets the bias voltage. I've used it with 5,6 V zeners, as these are reputed to be the quietest. But I can 't hear any difference with 9,6 or 12 V zeners, so I've stopped being anal about it
5,6 V zeners are the quietest, but the gain in S/R is probably undone by a rise in temperature or so...