It's regional, but in a pretty random way. An American character could say either of those without sounding strange. Goosebumps is one word, goose pimples is two.
According to every dictionary (that onelook.com checks) it's two words, or occasionally hyphenated. The Macquarie in my drawer agrees. As one word, it appears to apply to a book and TV series.
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The online dictionary I looked at says goosebumps is two words, but I didn't believe it!
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So it's two words? Hmm, maybe I should do a poll!
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