Hi there… just want to share this in case you need more then 15 post showing in sidebar in WP
/wp-includes/default-widgets.php
line 542: else if ( $number > 15 )
line 543: $number = 15;
line 591: <small><?php _e('(at most 15)'); ?></small></p>
Change the number "15" to the maximum number of posts each time you upgrade to WordPress you will have to make the changes again.
I hope can be usefull.