The traffic answer is all over the forums but in bits and peices. Basically each player will nominally use 2-3 kbytes/sec. This can change radically up to four times the traffic in any of these situations: players are in the same system in sight of each other, doing missions, around LOTS of NPCs. It also goes down radically if a player is docked. Upload is some 5-10% higher than the download. Of course this is for the original FL setup, your mileage with mods may vary.

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Earendil
SysAdmin of Boston Freelancer server
See server rules and server news for more info.

If Earendil doesn't mind I would like to draw a final line to what he mentioned. To run a server with 10 players online and be sure you don't get any lag due to lack of bandwidth you should have a 512 kbps upstream (kilobits per second = 64 kBps, kilobytes per second) and 128 kbps downstream for the server. Even fi you have this connection, you may also expect lag when you have many players online all of them doing dogfights (specially when it comes to PvP fights) due to CPU usage.

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3NET Freelancer Server Admin

See server forum here.
See server stats here.