Is there any constraints about number of different topics for Firebase Cloud Messaging in one app?
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 would seem that it takes a day for it to be visible in the Firebase Console, as per this post. Cheers! :D