If Howl of the Horde is cast before combat and then a creature attacks, is the next spell copied once or twice?

Nope. The Howl checks when it resolves to see if you've attacked with a creature that turn, not when you cast the next instant or sorcery spell. Since you're casting the Howl before combat, you haven't attacked with a creature yet, so the Howl trigger will only make one copy of the next instant or sorcery you cast.

Nathan Long
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