Twenty tries is scraping the farming surface. It took me exactly 100 tries to get Graf, and if I recall, there were some Nico streamers who tried 400+ times and still didn't get her.
On the flipside, there was another nico streamer who farmed a number of them for the planes.
Dated on November so means this is the first time for Graf to drop.....took me 5 damn days of sailing around easy difficulty and managed to clear event on the very last day.
Then she drop again on previous events....on much much EASIER map to farm......
20 tries ? It took me 76 tries to get Graf, 79 more to not find Kiyoshimo in the Fall Event and... 388 MORE times to find Sakawa in Winter's E-1. SS Hime is forever traumatized by my fleet (Almost sunk 450+ times in 2 Events if I not count the runs were I went for Akizuki in E1I or the retreats)