How to autocheck the existing folder and proceed further

EXAMPLE: In SOURCE, files will be placed in multiple folders as shown below:

C:\\users\A\B\010\100 Files
C:\\users\A\B\011\100 Files
C:\\users\A\B\012\100 Files

But in DESTINATION, files are copying as per below folder structure as shown below which is good and required in the same way only:

D:\\users\Y\Z\001\25 Files
D:\\users\Y\Z\002\25 Files
D:\\users\Y\Z\003\25 Files

  if (!int.TryParse(textBox3.Text, out int thresholdValue) || thresholdValue < 1)
            {
                // TODO: Display an error message to the user
                return;
            }

            string source = textBox1.Text;
            string destination = textBox2.Text;
            int totalFileCount = 0;
            int currentSubFolder = 0;
            int remainingFileCount = 0;
            string destinationFolder = null;

            ISet<string> extensions = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
{
    ".tif",
    ".tiff",
    ".jpg",
    ".jpeg"
};

            IEnumerable<string> files = Directory
                .EnumerateFiles(source, "*", SearchOption.AllDirectories)
                .Where(f => extensions.Contains(Path.GetExtension(f)));

            foreach (string sourceFile in files)
            {
                if (remainingFileCount == 0)
                {
                    // First file, or the sub-folder is full:
                    currentSubFolder++;
                    destinationFolder = Path.Combine(destination, currentSubFolder.ToString("D3"));
                    if (!Directory.Exists(destinationFolder)) Directory.CreateDirectory(destinationFolder);
                    remainingFileCount = thresholdValue;
                }

                string destinationFile = Path.Combine(destinationFolder, Path.GetFileName(sourceFile));
                File.Copy(sourceFile, destinationFile);
                totalFileCount++;
                remainingFileCount--;   
            }

            MessageBox.Show("All " + totalFileCount + " files are copied");