Does this mean they just start nibbling anyone and there will be meny werewolfs and npc be werewolfs too
Well... There are only two werewolves spawned for each neighborhood (including subhoods), so no there won't be many werewolves. They only bite Sims, not other animals.
If you don't want certain Sims to be bitten by these werewolves, there are multiple ways to prevent that from happening:
1. You can put a fence across the front of the lot and add a gate. Disallow pets to keep the werewolves out. Make sure there is no space somewhere in the front where an animal or a visitor might get through.
2. You can send your Sim to bed obviously.
3. Shoo or chase the pet until it leaves. I don't remember which one sends the visiting animal off the lot.
4.
DJS (jfade) has two wolf signs to allow or disallow werewolves on individual lots. You can buy the allow wolf sign from the Aspiration window and place it on the lot you want the Sim to interact with werewolves. Otherwise buy the disallow sign and place it on the lot, and no werewolves will show up. If they do, they just turn around and leave.
Easier to use these signs to control what lots you want the werewolves to visit and what lots you don't want the werewolves to visit instead of getting a mod that will affect the game globally (all lots).
The sky is the limit.
I am assuming that yes, the werewolves
will bite the Sim if the Sim is outside and not behind the fence.