How many Zodiac Killers are there?

In June 1998, a jury found Heriberto Seda to be the New York Zodiac Killer.
The trial began on 15 May 1998 in Queens, New York and lasted for five weeks. Seda pleaded guilty to three accounts of murder and several accounts of attempted murder.
He was sentenced to 152 years in prison, having to serve 83 years before being eligible for parole. It was agreed that, due to his age, Seda had nothing to do with the Zodiac crimes in California.