WP_Query with Metavalue

I would recommand to add meta_query in array param, try this

add_shortcode( 'sc_count_brands', 'sc_count_brands_code' );
function sc_count_brands_code($atts) {
  $values = shortcode_atts( array('category'  => 'Horology',), $atts );
  $args = array(
    'post_type' => 'brands',
    'post_status' => 'publish',
    'meta_query' => array(
        array(
          'key'     => 'br_category',
          'value'   => esc_attr($values['category']),
          'compare' => '='
        ),
    ),
  );
  $query = new WP_Query( $args );
  $countn = $query->found_posts;
  $buffer="<span class="magby">Featuring </span><span class="axiac">" . $countn . '</span><span class="magby"> brands</span>';
  wp_reset_postdata();
  return $buffer;
}