It depends how the server is set up. The owner of the server can change the size of the server to let more or less people join.
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As of the date of this answer, over 42 million people have created accounts on minecraft.net, the game's website, and over 8,293,494 have upgraded their accounts to premium and bought the game.
Today, 14th of Juny, 2011, a bit over 2.5 million people have bought the game. But I believe there are almost as many people playing minecraft illegally as those who play it legally. Therefore, about 5 million people in total. The total population in the world is about 7 billion. So... About 0.07% of the world play Minecraft.
as of may 11, 2013, 10,450,125 minecraft accounts have been purchased.