HA in a live environment must always be spread across DIFFERENT machines (i.e., nodes), else you introduce higher points of failure in your landscape. You can improve HA further by spreading those nodes across different (read independent) hardware kits and different data centers.
Note that clustering and HA are 2 different concepts/requirements, so you can have HA without clustering the nodes, unless your business case explicitly requires clustering (shared session states, shared caches, etc.). Read up online about stateful vs stateless clusters.
Multiple instances is only for a few cases that I’ve mentioned above - you must not use these in Test or Production environments.
KM
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