Standard 10-pin bowling was created as a legal loophole to bypass 19th-century laws banning a game played with how many pins?

Nine

Nine-pin bowling was a European predecessor of American tenpins, and the familiar loophole story says a tenth pin was added after ninepins was outlawed. However, the United States Bowling Congress says exactly why and when the extra pin was added remains unknown, so the story should be treated as folklore rather than established fact.