For servers (Host or VM) you can monitor availability, response time, volumes, disk space, up/down status of the node, etc - if you choose ICMP and WMI as the polling method. If you choose SNMP, you'll grab some better stats like network interface stats and environmental stats - along with many of the same items as the ICMP/WMI polling.
The WMI/ICMP polling method is super simple to set up - requiring only an IP address and an AD account with elevated privileges. The SNMP one takes a bit longer and requires more configurations such as community strings and services to be enabled and started.
You'll get better info from the SNMP polling in my opinion - interface stats are a big one for me (and most network engineers) and the WMI/ICMP polling just doesn't do it.
For the licensing portion it's a bit cryptic but here's how it breaks down (as was explained to me by SW sales) - Each element is broken down into 3 things (Volume, Interface, Node). If you purchase for instance a license pack for 1000 elements, you will be allowed to monitor 1000 Nodes, 1000 Interfaces, 1000 Volumes. But be advised 1 node can have many interfaces or many volumes which will whittle down your licensing very quickly - so choose wisely.
Hope that helps - And anyone else feel free to chime in and correct me if any of the licensing info is wrong.