Why Does Sauron Have Only 9 Fingers?

In the books, Gollum tells us the Dark Lord has only 9 fingers, but since he is a Maia manifesting a body and not strictly an incarnate being, why should this be the case?

For my review of David Rowe’s book, go here: https://www.youtube.com/watch?v=Gi2mJ6mFopM

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