How to interpret IoT quota: Subscriptions per second per account

The quota is per-region, per-account, and based on the MQTT SUBSCRIBE command--across all MQTT connections. Note that a SUBSCRIBE can contain one or more topics for subscription, so each topic (topic filter) is counted against quota. Note that exceeding quotas would normally happen during events such as all devices losing connectivity and reconnecting such as from an upstream Internet service provider failure. This can introduce a storm effect.

As noted, this quota is subscriptions-per-second across all MQTT connections making SUBSCRIBE attempts.

It's not called out in the documentation what AWS IoT Core will do when the quota is breached, but it would likely fail the SUBSCRIBE(s) being made that take place beyond quota (SUBACK with rejection). This is an adjustable quota, so I would recommend bench marking what the highest potential would be, then request a quote increase to provide a suitable buffer.

Also, in your device or application logic, it's good to add exception handling in case a subscription fails, such as SubscribeError. That could then be used to attempt a subscription retry (with back-off).