The PSA pop report shows a total of two Sovereign 150 Frank Isbell cards. The SGC pop report shows a total of five. There are other known copies, both raw and slabbed, including this PSA 2.5 that I bought two years ago from David Hall's collection at Heritage:
https://bit.ly/34ZeCeh But suffice it to say, the card is extremely rare. It's likely the rarest of all Sovereign 150 backs, and one of the rarest of all Sovereign backs. The published back-scarcity rankings don't matter much when it comes to this card.