string[]strs=textbox.Text.Split ('\n');
int a=(strs.Length%1000==0)?strs.Length:(strs.Length+1);
//存入的a文本个数
string[]str;
for(int i=0;i<a;i++)
{
   //strs分成a个数组str，每个数组存一个文本
   File.WriteAllLines("", str);
}

------解决方案--------------------

protected void Button1_Click(object sender, EventArgs e)
		{
			string aa = "dddddddddddddddddddddddd";
			TxtWrite(aa, @"f:\1.txt");
		}
		//=====================
		//描述：写入TXT文件
		//返回：void
		private void TxtWrite(string strToWrite, string filename)
		{
			StreamReader sr = new StreamReader(filename, Encoding.Default);
			int i = 0;//取得行数
			string s;
			while ((s = sr.ReadLine()) != null)
			{
				i++;
			}
			if (i > 10000)//大于10000行数的时候新建
			{
				//新建文件夹
				string FullFileName = DateTime.Now + ".txt";
				FileStream fs1 = new FileStream(FullFileName, FileMode.CreateNew);
				fs1.Close();
			}
			else//否则就是写
			{
				FileStream fs = new FileStream(filename, FileMode.Create, FileAccess.Write);
				StreamWriter sw = new StreamWriter(fs);
				sw.BaseStream.Seek(0, SeekOrigin.Begin);
				sw.WriteLine(strToWrite);
				sw.Close();
			}
		}

------解决方案--------------------
textbox？
C# code

   string yourfilepath="你要存储的文件路径";
            StreamReader reader = new StreamReader(yourfilepath);
            StreamWriter writer;
            int count = 0;
         //   string[] aa=new string[]{System.Environment.NewLine};
            count = reader.ReadToEnd().Split(new string[] { System.Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries).Length;
            //或 while(reader .ReadLine())count++;
            reader.Close();
            if (count > 10000)
            {
                int index = yourfilepath.LastIndexOf("+");
                int n ;
                int.TryParse(index == -1 ? "0" : (yourfilepath.Substring(index == -1 ? 0 : index + 1)),out n);//序号
                int indextemp = yourfilepath.Substring(0, index == -1 ? 0 : index).LastIndexOf("+");
                string temp = yourfilepath.Substring(0, indextemp == -1 ? (yourfilepath.Length - 1) : indextemp);
                string newfilepath = (n == 0 ? string.Empty : temp) + DateTime.Today.ToLongDateString() + "+" + (n + 1).ToString();
                writer = new StreamWriter(newfilepath);
            }
            else writer = new StreamWriter(yourfilepath);
            writer.WriteLine("你要存储的信息"）;
            writer.Flush(); 
            writer.Close();