问题:

Is there any constraints about number of different topics for Firebase Cloud Messaging in one app?

回答1:

Nope. Seeing that FCM has GCM as its core, there is no limit in the number of Topics for any app. There used to be a 1 million limit, but it was removed. You can refer to this Google Developers Blog for that.

Also, when creating a Topic in FCM, it could take up to 24 hours for it to be show up in the Firebase Console, as per this post. Cheers! :D

回答2:

No I don't think so!

Based on the publish/subscribe model, topic messaging supports unlimited subscriptions for each app. You compose topic messages as needed, and Firebase handles message routing and delivering the message reliably to the right devices. link

Seems like you can create topics when you need them!