<?php

$filenames = array(
    'Vacation (10).png',
    'Vacation (Me and Mom) (21).png',
    'Vacation (5) (1).png',
);

foreach ( $filenames as $filename ) {
    if ( preg_match( '/^.*\((\d+)\)/s', $filename, $matches ) ) {
        echo "$filename => $matches[1]\n";
    } else {
        echo "$filename => no match!\n";
    }
}