The core set for Zombicide: Black Plague contains the same number of Runners as Fatties, which is 14. Like the Fatties, they come in two poses. I have converted some of them, just so they don't all end up looking the same. Runners in Zombicide: Black Plague follow the same rules as they do in other games of Zombicide. They have two Actions per Turn, just one wound will kill one and they grant one Experience Point to the Survivor who kills each one. After all Zombies (including the Runners) have gone through the Activation step and resolved their first Action, the Runners go through the Activation step again, using their second Action to Attack a Survivor in their Zone or Move if there is nobody to attack.
I've said this before, but I do not like fast moving Zombies in games, books or films. I'm a traditionalist and I like my Zombies to be slow and shambling. That said, I am happy to accept Runners in the Zombicide games. They just seem to fit in so well and are always a dangerous threat to the Survivors whenever they appear. Go figure!