Run Code Only on the Main Query

add_action( 'pre_get_posts', 'wpfolks_adjust_main_blog_query' );

function wpfolks_adjust_main_blog_query( $query ) {
    if ( is_admin() || ! $query->is_main_query() || ! $query->is_home() ) {
        return;
    }

    $query->set( 'posts_per_page', 12 );
}