There is shortage yes in the sense that the heart didn’t come from just any pig. It is a specific, modified breed which made the procedure more likely to succeed. The donor pig was also probably raised in more sanitary conditions than most other pig would be.
There is not a huge supply of these transplantable pig hearts at the moment. But if the procedure proves to be viable and the long term consequences make it favourable we can scale up transplantable pig heart production.
The reason we are not transplanting pig hearts to thousands of patients right now is not because of any such supposed shortage though, but because the procedure is not proven yet. The experiment in question is set out to provide data to help answer that question.