If you're setting up a server and seeing that it doesn't reach its full capacity to hold players it's not because no one wants to go to your server. Here is how to find out what the maximum number of people you server can hold is

First go to speedtest.net and find out your UPLOAD SPEED. (remember upload speed is in mega BITS per second)

Each player thats comes to your server can use anywhere between 20 Kbps or 40kbps so for the sake of it lets assume 40kbps is the MINIMUM

Now lets say you have a 1Mbps upload speed to find what your server theoretically could hold do this ( Your upload speed / 0.04 ) in this case it's ( 1 / 0.04 ) = 25
25 theoretical players

This is if you maxed out your bandwidth which would create lag, connection problems etc etc for other players. To realstically find out how many players you could hold lets assume you only use 75% of your upload bandwidth which (Theoretical player * 0.75) so in out example it would be
(25 * 0.75) = 19

19 maximum players

Remember though this is for NON-DEDICATED SERVERS so basically a home connection or such. If you're running a dedicated server you probably know what you're doing and this doesn't apply

Is the relationship between players and bandwidth needed exponential or liner?
Ex: 3 players in my server. Uses 2.33kBps.
Yesterday: 32 players in my server, used 150kBps minimum.

What do you base the number 0.04 on?

Also, can you speak in bytes instead?

Edit: Don't forget when people initially join they need to receive the entire map. When someone joins, I usually end up uploading to that particular person at ~200kBps. It can take a long time for a person to get into your server if they are feeding off the sliver of bandwidth you have left, and I guess it may lag other users.

I had 20 players on my server using a total of 40KB/s, my max upload speed is 1000KB/s so it was not maxxed out x)

I hardly had any download speed showing up though.. I was pretty surprised it uses that little speed

You should post this information on the wiki, its quite helpful.

http://www.aceofspadeswiki.com/index.php?title=Main_Page