According to Project-GC, the best I've done is 38 mystery caches in a day, which was very likely one of the times I've gone out chasing solved puzzles on pieces of geo-art. The last time I got 5+ mystery finds in a day was in Ballarat on 4 December 2019, on the way home after a road trip to Tasmania. I did a bunch of challenge caches (listing attached) that day while the cuddle muggle was undertaking a bit of retail therapy.

Thanks for creating this locationless challenge Fitzy ... I'm quite enjoying researching my find records while we're cooped up at home.