WordPress Short code with additional parameter

First of all: Please read this:
http://codex.wordpress.org/Shortcode_API
Everything you need to know about shortcode parameters is in there.

Second: If you want to split content automatically into two or more columns, why don’t you use css?
http://www.w3schools.com/css/css3_multiple_columns.asp
works pretty good.

Happy Coding,
Kuchenundkakao