Here the answer given by Firebase support:
Firebase Cloud Messaging is free of charge, no matter the number of users and message pushes, just like Google Cloud Messaging was.
However, here are some of its product "limitation":
There's a limit of 1000 registration tokens used when using registration_ids parameter per send. So if you want to send 5000 messages to different users, you'll need to send the message in 5 batches. There's a limit of 100 messages that can be stored without collapsing. You can read more details here.
You may also encounter limitations such as sending too much messages per second. Aside from that, everything in FCM is free and unlimited.
We currently don't have an SLA for FCM since it's a free service. We could never guarantee the delivery of messages since we do not control the networks between our servers and the device. We also offer the same level of support for both our paid and free plans for this service. Kindly see our Pricing FAQ here, under "What kind of support will I receive?" for more information.
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