seb, there is no formula for that. Here , Here and Here you can find some information about it. Or the best thing to do IMO is install and configure asterisk as you’d like, and after that test with http://sipp.sourceforge.net/
The main problems in Asterisk appears when you need to process too many transcoding processes or many people using a same channel/bridge simultaneously (conference with too many people), and it’s generally solved with more CPU. Of course, if you have some bug on the way it’s a bug, not a hw problem. If you don’t have these situations, it’ll work like a charm with this hw that you posted.