public class Program
{

    public static void Main()
    {
        var path = @"C:\Users\tlamb\Downloads\AccessGUDID_Delimited_Full_Release_20231001";
        var input = "gmdnTerms.txt";
        var format = "gmdnTerms-{0:000}.txt";
        //var input = "device.txt";
        //var format = "device-{0:000}.txt";
        var size = int.MaxValue;
        var info = new FileInfo(Path.Combine(path, input));
        size = (int)Math.Min(info.Length / 10, size);
        Split(Path.Combine(path, input), Path.Combine(path, format), size);
    }

    public static void Split(string inputfile, string outputfilesformat, int maxSize = int.MaxValue, bool hasHeader = true)
    {
        int i = 0;
        int size = 0;
        StreamWriter outfile = null;
        bool firstFile = true;
        string headerLine = null;
        string line;

        try
        {
            using (var infile = new StreamReader(inputfile))
            {
                while (!infile.EndOfStream)
                {
                    line = infile.ReadLine();
                    if (hasHeader && headerLine == null)
                    {
                        headerLine = line;
                    }
                    if (size > maxSize)
                    {
                        if (outfile != null)
                        {
                            outfile.Dispose();
                            outfile = null;
                            size = 0;
                        }
                    }
                    if (string.IsNullOrEmpty(line))
                    {
                        if (outfile != null)
                        {
                            outfile.Dispose();
                            outfile = null;
                            size = 0;
                        }
                        continue;
                    }
                    if (outfile == null)
                    {
                        outfile = new StreamWriter(
                            string.Format(outputfilesformat, i++),
                            false,
                            infile.CurrentEncoding);
                        if (!firstFile && hasHeader)
                        {
                            outfile.WriteLine(headerLine);
                        }
                        firstFile = false;
                    }
                    outfile.WriteLine(line);
                    size += line.Length;
                }

            }
        }
        finally
        {
            if (outfile != null)
                outfile.Dispose();
        }
    }
}