First things first, head over to the Firebase Console. Log in with your Google account. If you don't have a Firebase project yet, you'll need to create one. Once you're in, you'll land on your Firebase project's dashboard.

On the left side of your dashboard, scroll down until you see the 'Grow' section. Click on 'Cloud Messaging.'

If it's your first time here, click 'Send Your First Message.' If you've used it before, you'll see a list of all your past and current messages.

To keep an eye on a Firebase Cloud Messaging subscription topic, you need to create one first. From the Cloud Messaging dashboard, choose the 'Topic' option when setting up your message audience.

Give your topic a name and then hit the 'Review' button. Check all the details, and once you're sure, click 'Publish'.

Firebase doesn't have a built-in feature to track the number of subscriptions to a specific topic. But you can still keep an eye on how your messages are doing.

In the Firebase Console, under 'Cloud Messaging,' you'll see all the messages you've sent. In the 'Used For' column, you can check if a message was sent to a specific topic.

Click on the message you want to dig into. This will take you to a 'Message details' page. Here, you can see analytics like the number of messages sent, delivered, opened, and more. Just a heads-up, these stats might take a bit to update after you send the message.

While Firebase doesn't let you directly monitor topic subscriptions, it does give you detailed stats on how your messages are performing. Regularly checking this can give you a good idea of how effective your messages are and how your users are interacting with your app.