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.

Johan van der Wijk

The Recent Posts Widget Plus plugin allows you to display a list of the most recent posts from all or a specific category or tag. It not only shows the post title, but also an excerpt of the post.

Recent Posts Widget Unlimited vs Recent Posts Widget Plus