var source = File.ReadLines(@"C:\MyText.txt");
File.WriteAllLines(@"C:\target1.txt", source.Take(1));
File.WriteAllLines(@"C:\target2.txt", source.Take(2));
File.WriteAllLines(@"C:\target3.txt", source.Take(3));
File.WriteAllLines(@"C:\target4.txt", source.Take(4));
//tüm satırlar
File.WriteAllLines(@"C:\target5.txt", source);