I don't know the number, but I know there were several prototype. The guitar begins as a thinline H-550. They added some Gretsch-inspired electronics, HRW pickups, and a Bigsby. David didn't like the sustain, so they put in a center block. That's the final prototype and the one I have. The next ones had signatures on the headstock and pickguard.
Pros:
Play well, sounds great, dramatic and beautiful blue
Cons:
Heavier than a H-555, although I have H-157s that weigh as much