TAG Query showing only 1 result

Try an expanded query as your current query is relying on the slug of the tag.

$args = array(
    'posts_per_page'   => -1, //get all posts
    'post_type' => 'post', //assuming your are not using a CPT
    'tax_query' => array(
        array(
            'tag' => 'homepage' //select your tag
        ),
     ),
);

 $casestudy_query = new WP_Query( $args );