How many concurrent active Skype calls/channels can I make using this SIP Skype gateway with a PBX such as Asterisk? You are limited by your available resources (memory, bandwidth and cpu). Here's the approximate per channel memory consumption and cpu usage for both supported multi channel platforms. Windows - 56-70mb - Approx 12% cpu with an AMD Athlon 2700 Linux - 52mb+ for SipToSis & Skype and 16mb+ for Xvfb - Approx 5% cpu with an AMD Athlon 2700 using snd-dummy. If you use a different method to create X sessions, your memory usage will be different.
Conservative Ram Calculation: (#channels x 100MB) + OS ram requirements MB + Other APP requirements MB = TotalRam MB
CPU usage above is with standard PCMU codec (30ms packets) and RFC2833 DTMF. You will have to experiment to find the limit for your environment. You can always expand your channel count by adding more boxes. There are no built-in limits. Use of a broadband internet service provider with sufficient bandwidth is required for multi channel operation.
I have run 10 channels myself - I do not have enough bandwith to run more. Some have run 40 channels. If you run 40+ channels I recommend a dedicated 64 bit quad core server with 8gb ram running linux.
There is a major problem on siptosis. Sometimes, your skype clients cannot be shown as online when viewed by other skype users. No reason why. But at the asterisk server, it is online.