1). Three per Lurker. So essentially, the total amount of Shroud Zones in play is 3 x Lurkers. Two Lurkers lets you have six at once.
2). The text for Scouring Whip states "Attacks made against targets within a Shroud Zone deal double damage." Is the target not within a Shroud Zone when you make the attack? Then it doesn't do double damage to them. Pulling someone who isn't in a zone into it with an attack doesn't retroactively make them in the zone when the attack was made.
You CAN pull someone into the zone with the whip, then apply damage, in the same way that you can order any simultaneous effects to occur in an order of your choosing...the same way you can choose to inflict knockback first, then damage, for example. So in this case, you can hit someone with the whip, drag them into the zone, this now makes them Shredded, and then you can apply the damage. But it'll only be the normal, baseline damage of the weapon, not double damage. That only occurs when someone is in the zone and THEN you attack them with it.