Same category

<?php
    // put all the category IDs of current post into an array
    $categories = get_the_category( $post->ID );
    $category_id_array = array();
    foreach ( $categories as $category ) {
        $category_id_array[] = $category->cat_ID;
    }

    // define how many thumbnails to show (at most)
    $thumbnail_max = 6;

    // grab all posts in the same categories as the current post
    $category_thumb_args = array( 'category__in' => $category_id_array, 'posts_per_page' => $thumbnail_max, 'exclude' => $post->ID );
    $category_thumb_query = new WP_Query( $category_thumb_args );

    // if there are posts loop through them
    if ( $category_thumb_query->have_posts() ) :
        while($category_thumb_query->have_posts()) : $category_thumb_query->the_post(); ?>


            ---- my stuff ---

        <?php endif;
        endwhile;
    endif;
    // reset the query since we used the_post()
    wp_reset_query();
    ?>