I am creating a solution that permit other app editor to sent notifications to their app through Firebase Cloud Messaging, and I want to develop the possibility to do A/B Testing. I came across the groups syteme of FCM and, if that works, it would be a good way to separate audiences in two or three groups, as it is possible to register several devices at the same time in a group. However it seems to be designed to represent the devices of a single user, which means a small number of devices. So my question is, can we still use FCM grouping with a large number of devices (in the tens of thousands) ? Or can they not be used this way?
No. Device Groups have a limit of 20 tokens per group:
The maximum number of members allowed for a notification key is 20.
High volume tokens in a group could be handled by Topics better.