If you’re going to deploy FastNetMon to really big network, you found best guide about it. You can find some basic numbers about required hardware in our install guide.
We have tens of deployments with traffic more than 2 Tbps in Netflow and sFlow modes. In Netflow/IPFIX or sFlow modes FastNetMon scales almost indefinitely. FastNetMon uses our own proprietary high optimised C++ packet parser.
In port mirror / SPAN mode FastNetMon can use all your available CPU resources. You will need 2-4 CPU cores for each 10-40G port. Also, you can use kernel side sampling introduced recently and it can lower hardware requirements to 1-2 CPU cores for 40G interface with decent network card.
For speed calculation, FastNetMon uses only single thread (limited by single CPU core performance) and it may affect you in case of really big number of IPv4 hosts (tens of millions or more).