Popular products are using different licensing policies which are difficult to compare or even estimate over time. In NetCrunch, when you monitor 50 machines, it's usually 50 licenses - no matter how many services, applications, and parameters you watch on each of them. However, this does not apply to network switch port interfaces which also represent network nodes, so monitoring of single 48 port switch with all their interfaces takes 49 licenses. The good news is that if you monitor the same nodes with other monitors or sensors, they do not consume more licenses.
NetCrunch is licensed by the maximum number of monitored elements - nodes or switch interfaces
This type of license is harder to predict. You need to have good knowledge about monitoring something that requires one or more probes, and these kinds of licenses scale poorly.
For instance: monitoring PING responses on 50 nodes take 50 licenses, but when you add additional service or start to monitor operating system, you will need an additional 50 licenses.
Every single monitored object (volume, interface) needs an additional sensor license.