Determining what post is displayed out of total posts found

You have all the necessary variables already at hand: $page is the current page number, and $portfolio->found_posts the total number of posts.

Both should be integers, but to be sure and to keep the code translatable I would use something like this:

$current = sprintf(
    _x( 
        'Page %1$d of %2$d.',  // default string
        '1 = current page, 2 = total pages', // context information for translators
        'your_text_domain'
    )
);

echo '<p>' . esc_html( $current ) . '</p>';

So nothing dangerous can happen, even if a translator injects some HTML.