Conditional based on number of specific custom taxonomy terms in archive.php

This should do the trick.

<?php 
// start with an empty array
$all_term = array();
if ( have_posts() ) {
    // loop through all posts, this could go outside of the main loop or be part of it, if placing outside the main loop be sure to reset the loop or namespace your queries
    while ( have_posts() ) {
        the_post(); 
        // add the list of terms for a given taxonomy to the $all_term array
        array_push($all_term, wp_get_post_terms($post->ID, 'my_taxonomy', array("fields" => "ids")));
    } // end while
} // end if

// remove duplicates from the $all_term array
$all_term = array_unique($all_term, SORT_REGULAR);
// output the result
echo count($all_term);
?>