Have implemented with the following way:

public class AppEventHandler : ApplicationEventHandler
{
    private const int FileAmountLimit = 3;
    private const string MaxAmountReached = "You have reached the Maximum Amount of the files. Please remove other files from the folder before upload.";

    /// <inheritdoc />
    protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        MediaService.Saving += MediaServiceOnSaving;
    }

    private void MediaServiceOnSaving(IMediaService sender, SaveEventArgs<IMedia> e)
    {
        var folders = sender.GetByIds(e.SavedEntities.Select(m => m.ParentId));
        if (folders.Any(f => f.Children().Count() >= FileAmountLimit))
        {
            if (e.CanCancel)
                e.CancelOperation(new EventMessage("File Upload Error", MaxAmountReached, EventMessageType.Error));
        }
    }
}

And it works like this: