You are trying to match 7-number-cities-starting-with-pa URL to ^([^/]*)-letter-words-starting-with- pattern in second rule.

These rules should work:

add_action( 'init' , 'se336474_rewrite_rules', 5 );
add_filter( 'query_vars', 'se336474_query_vars' );

function se336474_rewrite_rules()
{
    // single pattern for URLs: 
    //    site.com/cities-starting-with-pa/      => start_letter='pa'
    //    site.com/7-cities-starting-with-pi/    => start_letter='pi', length=7
    add_rewrite_rule( '^(?:([0-9]+)-)?cities-starting-with-([^/]*)/?$', 'index.php?page_id=340&start_letter=$matches[2]&length=$matches[1]', 'top' );

    //
    // OR for URLs:
    //    site.com/cities-starting-with-pa/         
    //    site.com/7-number-cities-starting-with-pi/
    //
    //add_rewrite_rule( '^cities-starting-with-([^/]*)/?', 'index.php?page_id=340&start_letter=$matches[1]', 'top');
    //add_rewrite_rule( '^([0-9]+)-number-cities-starting-with-([^/]*)/?', 'index.php?page_id=340&start_letter=$matches[2]&length=$matches[1]', 'top');
}
function se336474_query_vars( $vars )
{
    $vars[] = "start_letter";
    $vars[] = "length";
    return $vars;
}

You are trying to match 7-number-cities-starting-with-pa URL to ^([^/]*)-letter-words-starting-with- pattern in second rule.

These rules should work:

add_action( 'init' , 'se336474_rewrite_rules', 5 );
add_filter( 'query_vars', 'se336474_query_vars' );

function se336474_rewrite_rules()
{
    // single pattern for URLs: 
    //    site.com/cities-starting-with-pa/      => start_letter='pa'
    //    site.com/7-cities-starting-with-pi/    => start_letter='pi', length=7
    add_rewrite_rule( '^(?:([0-9]+)-)?cities-starting-with-([^/]*)/?$', 'index.php?page_id=340&start_letter=$matches[2]&length=$matches[1]', 'top' );

    //
    // OR for URLs:
    //    site.com/cities-starting-with-pa/         
    //    site.com/7-number-cities-starting-with-pi/
    //
    //add_rewrite_rule( '^cities-starting-with-([^/]*)/?', 'index.php?page_id=340&start_letter=$matches[1]', 'top');
    //add_rewrite_rule( '^([0-9]+)-number-cities-starting-with-([^/]*)/?', 'index.php?page_id=340&start_letter=$matches[2]&length=$matches[1]', 'top');
}
function se336474_query_vars( $vars )
{
    $vars[] = "start_letter";
    $vars[] = "length";
    return $vars;
}

You can use page_id=340 only if 340 is a page type post, in other cases a redirection to canonical URL will be made.

About first rewrite rule syntax [ (?:([0-9]+)-)? ] you can read here.