Which term is used to highlight that a storage administrator can add more than 2 storage systems to a cluster?

Scale up refers to increasing the resources of existing systems (adding more drives, memory, or CPUs to existing nodes), not adding new systems to the cluster.

Scale horizontal is not a recognized or standard industry term for describing this type of expansion in storage architecture.

Scale vertical is synonymous with scale up - it means adding resources to existing infrastructure rather than adding new nodes or systems to the cluster.

Scale out describes horizontal expansion by adding additional storage systems or nodes to an existing cluster, increasing total capacity and throughput without modifying existing hardware - this is the defining characteristic of clustered storage architectures like clustered ONTAP.