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 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.