The only way that I can think of to do this is to loop over them all, get the terms and build an array of artists where
$artists = array(
$artist_ID => $count
)
where the term ID for each artist is the array index and the you increment that array index for each one you find.
if (!isset($artists[$term_id])) {
$artists[$term_id] = 0;
}
$artists[$term_id]++;
Then the indexes are the unique values and the value of each index is the count.
