John mentioned that there were enough parts for about 250. So that is a maximum, but given how Grado usually keeps spare parts for repairs etc, I'd gather around 200? A good indication would be from the last buyer; he could check the number.
Regardless, this is by far the most limited of the Grado line. HP-1000's were 1000, perhaps nearly evenly split. THough HP-3's are technically the rarest (maximum 50 were ever made, and even then that number is estimated to be high) I would think that the fact that the majority of HP-3's were HP-2's anyway, means the PS-1's are truly the rarest. Half of what I would think amounts to either HP2 or HP1.