Increase number of posts in archive page

You can use pre_get_posts in your functions file to alter the query

function wpsites_query( $query ) {
if ( $query->is_archive() && $query->is_main_query() && !is_admin() ) {
        $query->set( 'posts_per_page', 100 );
    }
}
add_action( 'pre_get_posts', 'wpsites_query' );