Depends on your upload and your target streaming bitrate.
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 find your upload.
Use 2 to determine the number of listeners you can support at your selected streaming bitrate.
Jay (KXRM) also has a Bandwidth Calculator