generation of unique number

Here is an example to generate random number as well as string..

Hope it helps you to understand the logic..and you can create whatever condition you want.

StringBuilder builder = new StringBuilder();
        builder.Append(RandomString(4, true));
        builder.Append(RandomNumber(1000, 9999));

Generate random number

private string RandomString(int size, bool lowerCase)
    {
        StringBuilder builder = new StringBuilder();
        Random random = new Random();
        char ch;
        for (int i = 0; i < size; i++)
        {
            ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
            builder.Append(ch);
        }
        if (lowerCase)
            return builder.ToString().ToLower();
        return builder.ToString();
    }

Generate Random Number

private int RandomNumber(int min, int max)
{
    Random random = new Random();
    return random.Next(min, max);
}

You may also apply query to generate serial number

string sqlQuery = "select max(" + colName + ") + 1 from " + tblName;

If you still have any query don't hesitate to ask..
Happy coding.