Chars.cs

static void Main(string[] args)
{
    var lines = File.ReadAllLines(@"ナンバープレート一覧.csv", Encoding.UTF8).ToList();
    var places = lines.Skip(1).Select(x => x.Split(',')[2]);

    var chars = new List<char>();
    var charHashSet = new HashSet<char>();
    foreach (var name in places)
    {
        chars.AddRange(name.ToCharArray());

        foreach (var c in name)
        {
            if (charHashSet.Contains(c))
                continue;

            charHashSet.Add(c);
        }
    }

    File.WriteAllText("chars.txt", string.Join("", charHashSet.ToList().OrderBy(x => x)), Encoding.UTF8);
    File.WriteAllText("chars_rank.txt", string.Join("\r\n", chars.GroupBy(x => x).OrderByDescending(y => y.Count()).ThenBy(y => y.Key).Select(y => $"{y.Key}\t{y.Count()}")), Encoding.UTF8);
}