How to put multiple project_number/sender id in google-services.json

UPDATE: Going to refer to the official and recommended way in doing this instead of the hacky and unofficial approach to prevent/avoid unknown problems. From my answer here.

There is actually a part in the documentation about this topic:

Receiving messages from multiple senders

FCM allows multiple parties to send messages to the same client app. For example, suppose the client app is an article aggregator with multiple contributors, and each of them should be able to send a message when they publish a new article. This message might contain a URL so that the client app can download the article. Instead of having to centralize all sending activity in one location, FCM gives you the ability to let each of these contributors send its own messages.

To make this possible, make sure each sender generates its own sender ID. See the client documentation for your platform for information on on how to obtain the FCM sender ID. When requesting registration, the client app fetches the token multiple times, each time with a different sender ID in audience field.

Finally, share the registration token with the corresponding app servers (to complete the FCM registration client/server handshake), and they'll be able to send messages to the client app using their own authentication keys.

Note that there is limit of 100 multiple senders.

I think the confusing but important part here is:

When requesting registration, the client app fetches the token multiple times, each time with a different sender ID in audience field.

In other terms, you'll have to call getToken() passing the Sender ID and simply "FCM" (e.g. getToken("2xxxxx3344", "FCM")) as the parameters. You'll have to make sure that you call this for each sender (project) that you need.

Also, note from the getToken() docs:

This is a blocking function so do not call it on the main thread.

Some additional good-to-knows:

• It does not auto retry if it fails like the default one.
• It returns an IOException when it fails.