I am trying to do something similar to this block of code - but with tax terms rather than custom fields in a post

its a bit beyond me

    /**
 * Add an auto-incrementing Project ID field to Design feedback posts
 */
function auto_assign_ids( $post_id, $post, $update ) {
 
    // Only assign ID to new design posts
    if ( $post->post_status == 'publish' && $post->post_type == 'designapprovalsystem' ) {
 
        // get the most recent Project post
        $project_args = array(
            'numberposts'       =>   2,
            'post_type'         =>   'designapprovalsystem',
            'orderby'           =>   'post_date',
            'order'             =>   'DESC'
        );
        $projects = get_posts( $project_args );
 
        // get the project_id of the prior post
        //get the custom field value of a post
		$last_id = get_post_meta( $projects[1]->ID, 'job_number', true );
 
        // increment
        $last_id++;
 
        // set the project_id of the current post
		update_post_meta( $post_id, 'job_number', $last_id );
    }
}
add_action( 'save_post', 'auto_assign_ids', 100, 3 );