动态设置验证值如何？例如，首先获取图像记录计数，然后设置图像验证的值。

//Fetch the remaining count
$remaining_image_count = 3;
$image_count_validation = 'max:' . $remaining_image_count;

//Setting the validation value
'images.*'            => 'mimes:jpg,jpeg|max:10240',
'images'              => $image_count_validation,

如果我没理解错你的问题，你想检查一下数据库中是否最多有5个。

$count = 5 -  Media::where('post_id', $post->id)->count();
    $request->validate([
        'title' => 'required|min:20',
        'description' => 'required|min:50',
        'img' => 'array|max:'.$count,
        'img.*' => 'image|mimes:jpg,jpeg,png'
    ]);

如果这是你的问题，请具体点