The SF Playbill lists all characters - Harry, Albus etc., all the way down to tiny roles like a Ministry Official, the Station Master and the boys who bully Albus at Hogwarts, and then where it would read Ensemble, there is a large section that reads "Other Roles Played By". This includes a number of actors who are not playing named characters, and it is not made clear which of those actors are onstage ensemble members without named roles, and which are offstage swings. For example, it's easy to deduce that at least one of the two actors in the program who are primarily circus artists and aerialists are playing the dementor who flies over the audience at the end of part I, but there's nothing to indicate that for certain. Which, ya know, fine.
There are a total of 12 actors without a named role in the "Other Roles Played By" list. The show posted it's first night back curtain call on youtube, and I was able to count 7 actors in the curtain call who do not have a named role. Some of them were presumably covers, as there was at least one understudy on for that performance. Just trying to figure out how this all works, because god knows it's better than dealing with all my actual problems!