setting to see 25 attendees at a time

In the Zoom Desktop application go to Settings > Video.
At the bottom is an option called Maximum participants displayed per screen in Gallery View: 
You can select 25 or 49 participants.

Here’s an article that explains it well. There are some computer spec minimums that have to met, as well. 

https://support.zoom.us/hc/en-us/articles/201362323-Adjusting-your-video-layout-during-a-virtual-meeting