Variable showposts from administration

Hello world,
I have a problem with showposts.
Code:
…
$recent = new WP_Query('showposts=10'); while($recent->have_posts()) : $recent->the_post();
…

and I would like to change the number of “showposts” from the administration.

I found the instruction how to create widget in /wp-admin/
but i dont know how to create a simple form to change and save number of showed posts.

Thank you for your help.