Hi Cliff,
I should not have used the term "VM" as a synonym for "AWS instance". It was not a wise idea so let me slightly rework my reply.
A single row in SAP Cloud Appliance Library is a logical entity. This logical entity can be comprised on one or more (1 to n) SAP systems. Those SAP systems can be distributed among one or more (1 to n) AWS instances.
If you take the BW on HANA as an example and take a look at the details of the solution (click the corresponding row entry in the SOLUTION tab) then you will see that there a "Recommended VM sizes" section. This for example tells you how many AWS instances are started by CAL in the AWS account in order to have the logical entity (like the BW on HANA).
You can start and stop the individual SAP stacks if you like.
hdbadm is the so called SIDadm user for the HANA instance, a4hadm is the SIDadm user for the ABAP (A4H) instance. This is SAP standard.
Regards,
Hannes