Google Duo update further enhances support for group calls0
Google Duo is a pure example of a service that had to increase the number of people that could be involved in a group call from 8 to 32 in a matter of months. Even though Google Duo raised the group call limit from 8 to 12 back in March, the improvement was only marginal and required another boost at some point.
AndroidPolice reports that Google Duo is now getting an update that will further increase the group call capacity to 32. You'll know that you received the update if a pop-up shows up when you open the app to inform you about the change.
Keep in mind that this is a server-side update, so you don't have to do anything to get the improvement. If it's not there yet, give it a few more days since these roll-outs take a while to propagate.