Show thumbnails 1-12 of category x on any given page

Question

Have a look at the WP_Query codex, they have a range of parameters you can use, and example functions.

First setup your arguments, I have included 2 ways of doing this, the first being the easiest using the built in category parameters.

$args = array(
    'post_type'      => 'post',
    'post_status'    => 'publish',
    'posts_per_page' => 12,
    'category_name'  => 'x', // Use the category slug.
);

I prefer using the tax_query parameter, as it has given me less problems when using 3rd party plugins.

//Setup your arguments
$args = array(
    'post_type'      => 'post',
    'post_status'    => 'publish',
    'posts_per_page' => 12,
    'tax_query'      => array(
        'taxonomy' => 'category',
        'field'    => 'slug',
        'terms'    => 'x',
    ),
);

Lastly run the query and loop through it using a while loop.

//Run the query
$the_query = new WP_Query( $args );
 if ( $the_query->have_posts() ) {
    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        ?>
            <div>
                <a href="https://wordpress.stackexchange.com/questions/337796/<?php the_permalink(); ?>"><?php the_post_thumbnail(); ?></a>
            </div>  
        <?php
    }
    wp_reset_postdata();
}