只要将charNum改变就可以自定义设计生成随机数的个数。

//获取验证码的代码
        void GetValidateCode()
        {
            validStr = "";
            Random rd = new Random(); //创建随机数对象
            //产生由 charNum 个字母或数字组成的一个字符串
            string str = "0123456789";//共57个字符，除 l,o,x,I,O,X,1,0 的所有数字和大写字母
            for (int i = 0; i < charNum; i++)
            {
                validStr = validStr + str.Substring(rd.Next(10), 1);//返回0到56共57个
            }
        }

//随机码的生成获取5个随机数字

static void Main(string[] args)
        {
            string strRan = "0123456789";
            string str = "";
            System.Random rom = new Random();
            int strNums = 5;
            for (int i = 0; i < 8; i++)
            {
                for (int j = 0; j < strNums; j++)
                {
                    str+= strRan.Substring(rom.Next(10), 1);
                }

Console.WriteLine(str);
                str = string.Empty;

}
            Console.Read();

}

//获取5个随机的大写英文字母
  string strRan = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string str = "";
            System.Random rom = new Random();
            int strNums = 5;
            for (int i = 0; i < 8; i++)
            {
                for (int j = 0; j < strNums; j++)
                {
                    str+= strRan.Substring(rom.Next(26), 1);
                }

Console.WriteLine(str);
                str = string.Empty;

}