Firebase Cloud Messaging is completely free. The only limitations you'll encounter is pretty much related to the Lifetime of the Message and as you already know, the message size limit (4KB for data and 2KB for notification payload).
There is the Official Firebase Pricings Page where you can see the Cloud Messaging is included free for all their plans (Free and Premium) services, an estimated costs calculator, and the FAQs. But most of the limits are for the Database, Storage, etc.
- No. of Devices that can be added to a Device Group is 20.
- No. of messages that can be sent per day is unlimited, AFAIK. (See the lifetime of a message for the restrictions related to the message)
- There is no limit to the number of topics that can be created.
