Site Studio : adding content on a page "on demand"

Hello,

our Site Studio page templates display Contributor contents in colums.

Contributor users would like to add as many contents as they like in these columns. Is there a standard way to do this ? If not, what kind of customizations should we do ?

To my mind, we have to code the placeholders in the page templates, so I can't see how to do this ?

Thanks for your help,

Vincent