Why not use one body

Some Frankenstein movies have explained that this is to find the perfect parts... The hands of a craftsman, the brain of a genius, etc.
If you go back to the novel, things are a bit different. Frankenstein resolves to create a "being of gigantic stature" and characterizes the planned creature as "a new species". The actual work is discussed only peripherally. A mention of the "damps of the grave" and, in the same sentence, a reference to torturing living animals--I believe vivisection was a thing back then to learn about anatomy in animals. "I collected bones from charnel houses", says Frankenstein and a couple of sentences on: "The dissecting room and the slaughterhouse furnished many of my materials..." No further details are given and the actual creation is described in two, rather long sentences.