helpgeek

By default, the built-in Recent Posts widget in WordPress only allows you to 
display up to 15 recent posts max, even if you specify a larger number. This 
is hard-coded into the WordPress core so any modifications you make would have 
to be redone upon every WordPress update.
No longer!
The Recent Posts Widget Unlimited functions exactly as the built-in WordPress 
widget but without the 15-post restriction. You can now specify an unlimited 
number of recent posts to display without resorting to modifying the WordPress 
core.

Prasanna SP

This plugin creates a new widget which allows you to show recent posts by tags. The default Recent Posts widget in WordPress shows recent posts belonging to all categories. But this plugin gives you the option to show recent posts by tags. It is helpful if you are mainly using tags to categorize your posts.
Visit developer’s blog at <a href="http://www.prasannasp.net/" rel="nofollow ugc">prasannasp.net</a>
Support: Please post your support questions at <a href="http://forum.prasannasp.net/forum/plugin-support/wordpress-recent-posts-by-tags/" rel="nofollow ugc">Recent Posts by Tags plugin support forum</a>
Visit <a href="http://www.prasannasp.net/wordpress-plugins/" rel="nofollow ugc">this page</a> for more WordPress Plugins from the developer.

Recent Posts Widget Unlimited vs Recent Posts by Tags