As you may know that “AWS IoT Core supports 50 subscriptions per connection. AWS IoT Core might reject subscription requests on the same connection in excess of this amount and the connection is closed. Clients should validate the SUBACK message to ensure that their subscription requests have been successfully processed.”

However, if you didn’t check the SUBACK message, it could result in some issues. For example, sometimes it could incorrectly reporting success when you subscribes and exceeds number of allowed subscriptions limit. In this case, you may still see publish in event, but since the subscription was not actually successful so there is no publish out event at all.

You could look into wildcard subscriptions as one of the ways to reduce the number of subscriptions, to some extent.