Things You'll Need
Instructions
Download the "Minecraft" server files from the Minecraft.net downloads page.
Place "Minecraft_Server.exe" somewhere convenient, then double-click the program.
Close the server and check the location where you placed "Minecraft_Server.exe"; you'll see a small number of new files. Right-click the "Server.properties" file, select "Open With..." and open it in Notepad.
Change the "max-players=20" to a higher number. For example, "max-players=64."
Click "File" and "Save" and close the "Server.properties" file.
Restart your server by double-clicking the "Minecraft_Server.exe" file. More players will now be able to join your server.