How to get the Max Viewers of an Game out by Streamers, and specify wich one?

API
12

Is not required with helix.

Your URL is constructed wrong, as documented

it’s just name not game_name

This should work, and is a little cleaner and tests for errors than yours.

<?php

    $ch = curl_init('https://api.twitch.tv/helix/games?name=Minecraft')
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        'Client-ID: ' . $clientId
    ));
    $result = curl_exec($ch);
    $info = curl_getinfo($ch);
    curl_close($ch);

    if ($info['http_code'] == 200) {
        $resp = json_decode($result);
        if (json_last_error() == JSON_ERROR_NONE) {
            if ($resp->data && count($resp->data) == 1) {
                $gameID = $resp->data[0]->id;
                // go and do other stuff
            } else {
                echo 'Not one game returned';
            }
        } else {
            echo 'JSON Parse Error';
        }
    } else {
        echo 'Failed';
    }