How much overhead does the netpath agent cause on the source probe
Very little - I can't say more specifically than that. I have it running on some Windows servers that are RODCs, NAP, and a couple of other little things and are very underpowered - 1.5-2GB RAM total and low end CPU. I noticed no difference when installing the probe.
(And yes, upgrades for these servers are in progress.)
It will depend on how many services you are monitoring, the nature of the service, and your polling frequency. In general the more services you monitor, the more overhead. We monitor usage and based on that, show an estimate to the # of probes you can monitor without a major performance degradation. You will see that when you Create New Service:
So the 20 paths is based on some estimates of average probe performance.
Rick Matejka
In addition to what has already been stated, more specifics can be found in the Netpath Requirements -> Probe Computer section on page 188 of the NPM Admin Guide: https://support.solarwinds.com/@api/deki/files/10232/NPM_Administrator_Guide.pdf/..