Change number of tags to show in tag cloud

I was reading this article: http://codex.wordpress.org/Function_Reference/wp_tag_cloud

But im not sure where to edit that code.
I wish to only show 20 tag. It´s set to default 45 tags.

Where would I find that part to edit? In the wp installation or in the theme?

I´m using the default widget but a premium theme.