The number of channels that an audio track will record is determined by the channel count of its input bus. If you use the Master In bus (with its default setting of 2 channels) as the input bus for a track, the track will record two channels of audio data. If you're recording from a single channel source, such as a typical microphone or electric guitar pre-amp, you should create an input bus with a single channel, making sure it's connected to the external source you want to record, and then select it as the input bus in the track properties. This will result in a single-channel track which can be panned normally for mixdown to a stereo bus (such as Master Out), and avoids wasting disk space on empty or duplicate audio tracks.
For further details, see manual section 4.1. Routing--Connections, Ports, Tracks and Buses.