Final answer:
VMware High Availability (HA) automatically selects 2 datastores by default for datastore heartbeating to determine host failures and facilitate failover.
Step-by-step explanation:
In regards to High Availability (HA) within a virtualized environment, such as VMware, the number of datastores selected by default for datastore heartbeating is 2. When HA is configured, it automatically selects two datastores that are used for sending heartbeats, ensuring that a host is able to determine if another host has failed or is isolated from the network. This heartbeat mechanism provides a failover capability by which another host can take over the virtual machines from the failed host.