I\'m building an app where I am using Firebase Cloud Messaging. But I have some doubts. I want to know how many topics we could make in one app instance?
In official
Here, One app instance can be subscribed to no more than 2000 topics.
There is no hard limit on the number of topics an instance can subscribe to, this limit, like many other limits on FCM are in place to prevent abuse. You are unlikely to hit the limit if an instance is subscribed to less than a few thousand topics. If however this is insufficient and you are indeed hitting the subscription limit then I'd advise that you contact Firebase Support.
https://firebase.google.com/docs/cloud-messaging/android/topic-messaging
Some things to keep in mind about topics:
Topic messaging supports unlimited topics and subscriptions for each app.
You get too many topics error if you are subscribing for too many topics from a single app instance to check abuse.