Supposedly inserts a tab after each csv item in the source document, or a new line every _Columns_ number of items. The text documents currently appear to have new lines after each item. The relevant code starts on line 138.

Splitter.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CSV_Split
{
    class Program
    {
        static void Main(string[] args)
        {
            var s = new Splitter();
            s.Controller();
        }
       

    }

    public class Splitter
    {
        int Columns { get; set; }
        string sourcePath = @"C:\Users\DeepSky8\Downloads";
        string targetPath = @"C:\Users\DeepSky8\Downloads\SubDir";
        StringBuilder wholeFile = new StringBuilder();
        string newFileName;
        
        /// <summary>
        /// Coordinates user input, file reading, string creation, and new file creation.
        /// </summary>
        public void Controller()
        {
            while (NeedColumnNumber()) { }
            while (NeedFile()) { }
            NeedNewFileName();
            FinishUp();

        }

        /// <summary>
        /// Gets user input and parses & saves to class-level var. Use as while statement arg to ensure column number is accepted
        /// </summary>
        /// <returns></returns>
        bool NeedColumnNumber()
        {
            bool needNumber = true;
            Console.WriteLine("How many columns should the resulting text contain?");
            var entry = Console.ReadLine();
            var result = 0;

            if (Int32.TryParse(entry, out result))
            {
                Columns = result;
                needNumber = false;
            }
            else
            {
                Console.WriteLine($"{entry} didn't parse as an integer. Please try again.");
            }

            return needNumber;
        }

        /// <summary>
        /// Gets name of file to parse, including extension. If this file exists, reads the file into memory, stored in wholeFile.
        /// </summary>
        /// <returns></returns>
        bool NeedFile()
        {
            bool needFile = true;
            Console.WriteLine("What is the file name, including extension.");
            var fileName = Console.ReadLine();

            string sourceFile = System.IO.Path.Combine(sourcePath, fileName);

            if (System.IO.File.Exists(sourceFile))
            {
                var file = System.IO.File.Open(sourceFile, System.IO.FileMode.Open);
                var isDone = false;
                var data = new byte[1024];
                var totalBytesRead = 0;


                while (!isDone)
                {                    
                    var bytesRead = file.Read(data, 0, 1024);
                    totalBytesRead += bytesRead;
                    isDone = bytesRead < 1024;
                    wholeFile.Append(System.Text.Encoding.ASCII.GetString(data, 0, 1024));
                }
                
                needFile = false;
            }
            else
            {
                Console.WriteLine("Source does not exist!");
            }


            return needFile;
        }

        /// <summary>
        /// Gets user input to name the new file, and stores it in a class-level var.
        /// </summary>
        void NeedNewFileName()
        {
            Console.WriteLine("What should the new document be called?");
            newFileName = Console.ReadLine() + ".txt";
        }

        /// <summary>
        /// Splits the file in memory on commas, and inserts a tab after each item. Instead inserts a new line after the number of items indicated by Columns. Appends all this to a document, created if it didn't exist
        /// </summary>
        void FinishUp()
        {
            var isDone = false;
            var pointer = 0;
            var splitFileContents = wholeFile.ToString().Split(',');
            string[] eachLine = new string[Columns];
            string[] amendedLines = new string[Columns];
            if (!System.IO.Directory.Exists(targetPath)) { System.IO.Directory.CreateDirectory(targetPath); }
            string destFile = System.IO.Path.Combine(targetPath, newFileName);
            int remainderArray = (splitFileContents.Length % Columns);


            while (!isDone)
            {
                try { Array.Copy(splitFileContents, pointer, eachLine, 0, Columns); }
                catch (System.ArgumentException)
                {
                    if (!(remainderArray == 0))
                    {
                        Array.Copy(splitFileContents, pointer, eachLine, 0, remainderArray);
                    }
                }

                for (int i = 0; i < Columns; i++)
                {
                    if (i == Columns)
                    {
                        amendedLines[i] = String.Concat(eachLine[i], "\n");
                    }
                    else
                    {
                        amendedLines[i] = String.Concat(eachLine[i], "\t");
                    }
                }

                pointer += Columns;

                System.IO.File.AppendAllLines(destFile, amendedLines);
                
                if (pointer >= splitFileContents.Length)
                {
                    isDone = true;
                }
            }






            
        }


    }

}