How do I adjust Gallery View to show all thumbnails?

[MikeCannon]

Today I conducted a meeting for 20 participants using Gallery View. However, I could see only 12 thumbnails on the screen at a time. I had to scroll to the right to see the additional participants. How do I adjust Gallery View to see all 20 participants? (I can't find anything about this on the site after an hour of searching.)

Thank you!!!