List related terms + taxonomies

You probaby want get_terms().

WordPress keeps a count of the number of posts that use a taxonomy and you can filter your query using it:

// get all the terms in the 'category' taxonomy
$terms = get_terms('knife');
// list all the terms that have at least one post
foreach ($terms as $term) {
    echo "{$term->name} has {$term->count} posts."
}