That said, I think I prefer the "#1 in a series" more than "#1 of a series". I would be more likely to use "#1 of..." when followed by a specific number, such as: "#1 of 5", whereas I would be less likely to use "#1 in 5". "#1 in...", using in followed by an abstract quantity/collective sounds better to me, but that may just be my subjective ... More @Wikipedia
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