Increase returned number/pages on pagination

Refer to the Codex page for paginate_links for all accepted arguments.

The two you want to modify are mid_size and possibly end_size.

$output .= paginate_links( array(
    'base'      => str_replace( $big, '%#%', esc_url( get_pagenum_link( $big ) ) ),
    'format'    => '?paged=%#%',
    'current'   => max( 1, get_query_var('paged') ),
    'total'     => $wp_query->max_num_pages,
    'mid_size'  => 8 // number of page links to display on either side of current page

) );