That depends on what you mean by a bone - snakes have very long tails and each tail has a set of ribs, so I suspect that among vertebrates that actually have bones that it would be snakes or caecilians (snake-like salamanders) or something similar. However, for the largest number of hard parts, the title has to go to an echinoderm of some sort (starfish, sea urchins, etc.), whose bodies are composed of thousands of little calcium carbonate plates. One extinct class of echinoderms, the blastoids, had MILLIONS of these plates!
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