How how do I share my usb printer on a LAN network of 20 compurets?

You can use "Windows Printer Sharing" that's built into Windows, but your computer must be on for anybody to print. Search and you should get plenty of answers.

You can use a "Printserver". There is one in the URL below.

-or-

You can use a generic USB serial server. This can also share scanners and USB disk drives too. One model supports 2 connectors and 4 ports and the other 4 connectors and 8 ports. A combi printer/scanner would contain 2 devices with 1 port.

Keyspan | Products | USB Products