How to increase maximum number of sidebars support?

Just tried it on a test install with twenty-something sidebars, all works fine. Note that id parameter should be unique between all of them. name doesn’t have to be, but it’s nice to keep it unique as well so as to avoid confusion in the admin UI.

register_sidebar( array(
    'name' => 'twenty-three',
    'id' => 'sidebar-23',
    'description' => 'dummy description',
    'before_widget' => '<aside id="%1$s" class="widget %2$s">',
    'after_widget' => '</aside>',
    'before_title' => '<h3 class="widget-title">',
    'after_title' => '</h3>'
) );