Add to mods.php

// count mastered decks
function mastered_count($tcg) {
    $database = new Database;
    $sanitize = new Sanitize;
    
    $tcg = $sanitize->for_db($tcg);
    $tcginfo = $database->get_assoc("SELECT * FROM `tcgs` WHERE `name`='$tcg' LIMIT 1");
    $tcgid = $tcginfo['id'];
    
    echo $database->num_rows("SELECT * FROM `collecting` WHERE `tcg` = '$tcgid' AND `mastered` = '1'");
}

Add to Game’s PHP File

<?php mastered_count('<TCG NAME>'); ?>