there was a MMORPG open source release( http://www.blitzbasic.com/Community/posts.php?topic=36871 ), so I guess - yes, with a dedicated server. Don't know what you mean by Blitz3D server. If you are talking about streams in Blitz, then I guess you can have about 64 thousand simultanous strams in theory, but practically the amount of data and bandwidth/datatroughput of your connection will limit this.
network question
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