There is no sixth replicant; Bryant's dialogue gets it wrong.

An early version of the script called for a character called "Mary" (Stacy Nelkin was even cast in the role). The character's scenes were dropped due to budget constraints, but one reference to the extra replicant had already been recorded and survived the changes, creating a small continuity error in the process - and giving birth to a popular theory: namely, the idea that Deckard might himself not only be *a* replicant, but actually be part of Batty's group of escaped replicants! This would make Deckard the sixth replicant! The strange thing is that Scott had Walsh redo the line for an overdub but then decided not to use it.

According to the theory, Deckard would have been captured and "reprogrammed" (false memory implants, like they did for Rachael). The fact Roy Batty calls him by name even though they've supposedly never met before might suggest Batty knew him, thereby giving impetus to this theory, although there are other equally valid ways he would find this out, such as Roy hacking into the police Esper, overhearing his name or just simply that Deckard has a well-known reputation as the ace Blade Runner. "Aren't you the good man?" In fact there are so many holes in this particular theory that many Deck-a-Reppers acknowledge it as just a continuity error.

Incidentally, the fifth replicant, (the one that got fried), was called "Hodge".