Limit of Bulk Firebase Cloud Messaging

Solution 1:

According to this answer, the FCM limitations are similar to GCM.

From the GCM documentation:

...It must contain at least 1 and at most 1000 registration tokens.

So you're presumably able to send each message to 1000 devices. Since you're not limited in number of messages, you can send the same message 8 times, to 8 different lists.

Solution 2:

@NeriaNachum's answer is referring to sending messages to multiple recipients limit when using the registration_ids parameter. From the FCM docs (which I think is what you were looking for):

This parameter specifies the recipient of a multicast message, a message sent to more than one registration token.

The value should be an array of registration tokens to which to send the multicast message. The array must contain at least 1 and at most 1000 registration tokens. To send a message to a single device, use the to parameter.

Multicast messages are only allowed using the HTTP JSON format.

However, if you use Topics Messaging instead, there is no limit. Just make sure that each of the user you intend to send a message to is properly subscribed. If you have your own App Server, you can subscribe multiple tokens using the Instance ID API (see my answer here).