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SiRFstarIII does have the ability to track up to 22 satellites at once. However, with the current constellation there will be more than 12 satellites visible much less than 0.01 % of the time, so the current implementation of the receiver's software only supports up to 12 satellites at once.
You asked an interesting question about the difference between a satellite and a channel. A channel typically represents a subdivision of the hardware dedicated to tracking a satellite, but that definition goes away in SiRFstarIII. In reality, SiRFstarIII has no channels. Rather it has a tracking engine that tracks whatever it is told to track at any given instant. Most receivers track in real time, meaning as the stream of 1s and 0s come in from the RF section, the hardware assigned to track a satellite compares that incoming stream with another stream created inside the receiver and adjusted to match what that satellite should be sending. When the streams match sufficiently, we have a track. SiRFstarIII operates in faster than real time, much faster in fact. But of course that means it must work in the past. It captures a snapshot of the streams of 1s and 0s in a buffer. When the buffer is full the receiving hardware switches to another buffer to collect the next samples while the tracking hardware starts processing the filled buffer. That tracking hardware repeatedly processes the buffer for each satellite it is tracking -- if it is tracking 8 satellites it processes the buffer 8 times, once for each satellite. The tracking hardware is fast enough that it could process the buffer up to 22 times before the other buffer was full and the receiving hardware would need to switch and start filling this buffer again.
Since there are only enough satellites visible at one time to justify 12 "channels" (areas in memory to collect processing results, really), and since currently defined SiRF messages only support 12 satellites, SiRF chose at this time to only implement 12 parallel tracks. When circumstances justify it, we will expand things. Note also, that as processes improve so that we could speed up the chip, that 22 satellite limit will also go up.
Carl - SiRF Customer Support