How to list more than 15 recent posts?

/wp-includes/default-widgets.php around line 537

$title = apply_filters('widget_title', empty($instance['title']) ? __('Recent Posts') : $instance['title']);
if ( !$number = (int) $instance['number'] )
$number = 10;
else if ( $number < 1 )
$number = 1;
else if ( $number > 15 )
$number = 15;

I think if you change 15 to whatever number you need, that might do it. This is a core file, so use caution, make a backup first, and be aware that you will need to re-edit after an upgrade.

There may be a way to apply a filter to over-ride the settings from a theme file, but I’m afraid that’s a little above my head.

Thank you! I was searching all the widgets.php and couldn’t find it.

You’re welcome. Careful with the core files… make a backup just in case!