Mitchell is correct... Its based on usage. Sites like the ones around Sioux Falls have more voice channels to handle more simultaneous traffic. Its the same here in MN on our ARMER system, most sites have a minimum of four, one control, three voice, one of the voice channels even double as an alternate control channel. Some sites have 4-5 voice channels.
A little background on trunking that may help you to understand. When a user keys up it sends data to the controller on the control channel, the controller then tells the radio which channel it should transmit on as well as telling all other users in that talkgroup what channel their radio should listen to for the transmission. Thats how it handles multiple conversations at once, as they come in the controller sends them off to one of the voice channels, which is why in a densely populated area like Sioux Falls where all the agencies are on the statewide system and could potentially be at the same time they need more voice channels.
This is also why trunk tracking is as sophisticated as it is and the cost of the scanners so high, because they have to handle the data stream the same way the radios do, listen for the talkgroup info and switch to the correct voice channel.