I'm guessing that the exact answer depends on which streaming method you choose. From the wiki, it seems to me that DreamHost support Quicktime, Flash Video, Real Media, and Windows Media streaming.
DreamHost wiki article on http streaming of Real Media and Windows Media formats
DreamHost wiki article on Quicktime streaming.
DreamHost wiki article on Flash Video streaming.
Each of these players has different performance characteristics. The bandwidth requirements will vary based on the bitrate of your underlying video.
Have you considered contacting DreamHost pre-sales support at the following url? If you give them a sample bitrate and format, I'm sure they could give you an estimate of how many simultaneous streams they could support before it would create a problem on your server.
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