I need to add a filter to prepend the term ‘National – ‘ to the post title if the post is tagged to multiple states

Your “states” appear to be terms in a custom taxonomy. In the code below I’ve assumed the taxonomy’s name is my_state; you’ll need to make sure that’s updated to match the actual taxonomy name.

add_filter( 'the_title', function( $title, $id ) {
    // Gets the list of 'states' the post belongs to.
    $states = wp_get_post_terms( $id, 'my_state' );
    // If there's more than 1 state, prepend 'National - ' to the title.
    if ( 1 < count( $states ) ) {
        $title="National - " . $title;
    }
    return $title;
}, 10, 2 );

References

• wp_get_post_terms()