I think we need to bit a careful with those numbers. Whoever wrote the blog you refer to claims "up to" 10 concurrent calls and then he gives a reference to his own forum
http://sourceforge.net/p/raspbx/discussion/general/thread/4975db40/#eb2aThis is a forum thread where a contributor claims 10 concurrent calls and, fortunately for us, posts an image of a couple of FreePBX front screens which show us some stats. If you're still interested in this then I suggest you grab that image from the forum and have it in front of you as you read on. The image contains 2 screenshots, side-by-side, of the box running 23 "calls" and 12 "calls". So far so good. Note the left hand screen Load Average (the highlighted CPU info is irrelevant). It is showing 1.88. In other words, the box cannot cope at that load (by almost a factor of 2). In the second image we see the load average at .58, not great, but coping at an apparent 12 calls.
But look at "total calls" and "total channels" values - they are the same value in each screen (24 in the left and 12 in the right). So in the left screen its doing 24 calls and in the right screen, 12... right? Nope. In a "real" call, a minimum of 2 channels is used; one into Asterisk from the phone and one out of Asterisk to the destination. Asterisk "Bridges" calls, that's what it does. So what can we conclude from this? It's very likely that the calls in this case are all running into a conference room (so there's no need for an extra leg). They may be running into multiple conference rooms, I can't tell and the author doesn't say.
Putting it all together. While the conference bridge itself can be quite hungry, which needs to taken into account, if we accept the 12 channels as being achievable then that means the box can handle 6 concurrent calls. However, when you add the real world cpu spikes caused by call start up and routing (which can be quite large with dialplan generators like FreePBX) then I would set a safe, achievable number at no more than 4 or 5.
Kind Regards
S
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