Staggered Release with Tons of Carriers
You shouldn't expect Apple to get the iPhone 6s and iPhone 6s Plus out to everyone on the first release date though.
While you can count on the devices landing for a number of key regions right off the bat, Apple almost always uses a staggered release approach that allows its supply chain to catch up to demand. Apple's initial focus is usually on major markets like the United States, Japan and countries in Europe. It typically takes a few weeks for the rest of the world to catch up.
We'd also expect tons of different carriers to stock the iPhone 6s. This includes major carriers like AT&T and Verizon and smaller carriers like C Spire. The iPhone is one of the most widely available smartphones and that shouldn't change this year with Apple reportedly putting in record numbers of orders.