I just created a custom registration question with multiple choice. It allowed me to enter 20 choices, but when I went back in to edit, it allowed me to add more than 20 choices, but failed when I tried to save it. Can you make it so I can not try to add more then 20? I am using Firefox 84.0.2 on Mac OS 10.13.6.
I created a question with 20 possible choices, then went to the registration page and looked at the question. I could see only 10 of the 20 choices in the drop down list. I could not see a way to see choices 11 through 20. That was using FireFox on a display set to 720p. I moved the web browser window to another display that is set to 900 lines, and I could see all 20 choices.
When I tried in Safari web browser on the 720p screen, I could see all 20 choices. So the issue seems to be when using Firefox on a 720p screen.