Count posts per taxonomy else change taxonomy if less than x number

I’m guessing you are using a tax_query here.

If you try to query multiple categories or taxonomies, the terms field expects an array

$args = array(
    // other query arguments
    'tax_query' => array(
        array(
            'taxonomy' => $taxonomy,
            'field'    => 'id',
            'terms'    => array($cats_or_tags),
        ),
    ),
);