The document says:

Topic messaging supports unlimited subscriptions for each topic. However, FCM enforces limits in these areas: One app instance can be subscribed to no more than 2000 topics. If you are using batch import to subscribe app instances, each request is limited to 1000 app instances.

I already know what the following part means:

Topic messaging supports unlimited subscriptions for each topic. However, FCM enforces limits in these areas: One app instance can be subscribed to no more than 2000 topics.

But I'm not sure about the following part:

If you are using batch import to subscribe app instances, each request is limited to 1000 app instances.

Does this mean, for example:

If there are 5000 subscribers for a specific topic called "123", and the user sends a notification to this topic, does this last part of the document refer to breaking the limit? Or is it related to something else?

The document says:

Topic messaging supports unlimited subscriptions for each topic. However, FCM enforces limits in these areas: One app instance can be subscribed to no more than 2000 topics. If you are using batch import to subscribe app instances, each request is limited to 1000 app instances.

I already know what the following part means:

Topic messaging supports unlimited subscriptions for each topic. However, FCM enforces limits in these areas: One app instance can be subscribed to no more than 2000 topics.

But I'm not sure about the following part:

If you are using batch import to subscribe app instances, each request is limited to 1000 app instances.

Does this mean, for example:

If there are 5000 subscribers for a specific topic called "123", and the user sends a notification to this topic, does this last part of the document refer to breaking the limit? Or is it related to something else?

• firebase
• firebase-cloud-messaging

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The limit described in your last quote is related to how many tokens can be added to a topic subscription in a single API call. This is not a limit to how many tokens can be subscribed in total (which, per the first quote, is unlimited), just a limit on how many can be added (or removed) per API call.