How much bandwidth do I need to stream audio ?

The amount of bandwidth depends on the parameters you select on the Instreamer regarding encoding, quality and protocol. For MP3 the range is between 30 and 330 kbps.
Example: At CD quality, each stream is approx 192kbps. FM Stereo is 128kbps.

If you want to do a linear feed (no perceptual coding), a 44.1 kHz PCM stereo stream will consume most of a 1.544 mbit T1 line. See Bit Rate Calculation for bandwidth consumption of audio streaming with Barix applications.