Very confusing, I think AWS still doesn't unify their wording yet.

A shard (in the API and CLI, a node group) is a hierarchical arrangement of nodes, each wrapped in a cluster.

https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/CacheNodes.NodeGroups.html
AWS historical picture calling nodes <-> clusters equivalent
So the API documentation page now is using "nodes" instead of "clusters", but IMO that adds even more confusion reading through this parameter :

NumCacheClusters

The number of clusters this replication group initially has.

A Valkey or Redis OSS (cluster mode disabled) replication group is a collection of nodes, where one of the nodes is a read/write primary and the others are read-only replicas. Writes to the primary are asynchronously propagated to the replicas.