When you deploy your CloudHSM, it is deployed as a cluster and, by default, this cluster size is 6 per account, per region. However, you can configure your cluster to have a single HSM all the way up to 28 HSMs. The more HSMs you have, the better the performance will be. To avoid complications with key synchronization, AWS CloudHSM manages that for you. If you add additional HSMs to your cluster after the original creation, AWS CloudHSM will take a backup of all of your users, policies, and, of course, your keys, and then deploy that backup onto the new HSM within your cluster.
For additional resiliency and high availability, you should place your HSMs within your cluster in different Availability Zones within your region. The cluster architecture of AWS CloudHSM can be summarized as shown here:
One important point to note is that when you configure your cluster and you specify your subnets/Availability Zones as locations, what in actual fact happens is an Elastic Network...
