Check out the Sticky post titled...
SHOUTcast: Important Info & Answers to FAQs. READ ME!
Scroll down a bit and look for the section titled...
1. DNAS Stuff -> Bandwidth -> DSLReports Bandwidth tester(Only be concerned with the UPLOAD bandwidth)
2. DNAS Stuff -> Bandwidth -> CasterClub's Listener Calculator
3. DNAS Stuff -> Bandwidth -> CasterClub's Bandwidth Calculator
Use 1. to determine your upload. Use your upload and 2. to determine the number of listseners you can support at the bitrate you want to stream at.