https://www.goldderby.com/article/2019/agt-champions-superfans-voters-answers-nbc-americas-got-talent-news/
The Superfans were not in the audience watching the acts perform along with judges Simon Cowell, Heidi Klum, Mel B and Howie Mandel. Instead, the “America’s Got Talent” rep explains that “this elite group of individuals watched each show taping off-camera in a controlled environment within the show’s venue.” While the exact number of voters was not disclosed, they did confirm earlier reports that the Superfans are “not necessarily the exact same group of individuals voting for each episode, but there was some cross-over from week-to-week, similar to what you would find with any voting show.”
This is what they claim in the article.