It depends on what the author writing about them wants.
In some works of fiction, the forms a shapeshifter can take are limited only by his or her imagination and individual skill level. (e.g. transformation jutsu in Naruto)
In other stories, a shapeshifter is limited by what it has encountered. For example, the X in Metroid Fusion can take the forms of creatures whose DNA they have absorbed.
Other shapeshifters may be liited to a given set of forms, such as with Ben from the TV show Ben 10 or Dax frome the Shapeshifter book series. Still others may only be limited to one form into which they can shift, such as the animagi in Harry Potter.
Werewolves are shapeshifters
Yes. Apeiromorph does mean infinite forms in Greek.
Bruce Coville's Shapeshifters was created in 1999.
weightlifters
There are several shapeshifters in X-Men, but perhaps the best known is Mystique.
Yes. Shapeshifters are polymorphic because they can change into any form they want to.
It depends on the lore or mythology you're referring to. In many stories, shapeshifters are not immortal but have the ability to change their form. However, there are also versions where shapeshifters can live longer lives or have a form of immortality.
Shapeshifters and werewolves ;)
Just one: Beorn.
Yes, because shapeshifters are werewolves
Vampires and werewolves/shapeshifters.
Shapeshifters - Helter Skelter Lyrics