You stated "I'll be recording all of the instruments myself one at a time." So, you will only need one Input... Thus, you will record a guitar track, then possibly another guitar track, a bass line and vocals. Each Instrument used including the microphone will only require one Input, but make sure you have your interface has 1/4" inputs as well as XLR inputs for the mic. I would assume they all do, but I only have experience with an old old MBox. When you state, "does a two channel interface mean I can only record a left and a right guitar (two tracks)?," with the experience I have No would be the answer. A left and a right guitar makes It sound as if It's stereo, you usually call it like guitar one, guitar two. But for example with the old old version of pro tools I used you begin your new session by building tracks. You must have a stereo master channel, and then you can have many audio tracks. Your limit to audio tracks are part of the software as the amount of inputs are part of the hardware. Does this make sense? I could help you more, but rather than just blurt out info... would need more details.