documentclass[standalone]
   usepackage{tikz}
   begin{document}
   begin{tikzpicture}[scale=0.2]
   coordinate (O) at (0,0);
   fill[red!70] (O) circle (20);
   fill[white] (O) circle (19);
   fill[yellow!70] (O) circle (18);
   fill[white] (O) circle (17);
   fill[red!70] (O) circle (16);
   fill[white] (O) circle (15);
   fill[yellow!70] (O) circle (14);
   fill[white] (O) circle (13);
   fill[red!70] (O) circle (12);
   fill[white] (O) circle (11);
   fill[yellow!70] (O) circle (10);
   fill[white] (O) circle (9);
   fill[red!70] (O) circle (8);
   fill[white] (O) circle (7);
   fill[yellow!70] (O) circle (6);
   fill[white] (O) circle (5);
   fill[red!70] (O) circle (4);
   fill[white] (O) circle (3);
   fill[yellow!70] (O) circle (2);
   fill[white] (O) circle (1);
   end{tikzpicture}
   end{document}

documentclass{standalone}
usepackage{tikz}

begin{document}
begin{tikzpicture}[mystyle/.style={circle,draw,fill=none,minimum size=20, line width = 8pt}]
  foreach x in {1,3,5,7,9,11,13,15,17,19}
node [mystyle,  minimum size = x cm, color =red!70]  (2) at (0, 0) {};
  foreach x in {2,4,6,8,10,12,14,16,18, 20}
node [mystyle,  minimum size = x cm, color =yellow!50]  (2) at (0, 0) {};
end{tikzpicture}
end{document}

using SkiaSharp; // needs skiasharp nuget
using System.Diagnostics;


class ConcentricCircle
{
    static readonly SKPaint yellowStroke = new SKPaint
    {
        Style = SKPaintStyle.Stroke,
        Color = SKColors.Yellow,
        IsAntialias = true
    };


    static readonly SKPaint redStroke = new SKPaint
    {
        Style = SKPaintStyle.Stroke,
        Color = SKColors.Red,
        IsAntialias = true
    };

    static readonly float scale = SKDocument.DefaultRasterDpi / 2.54f; // dots per cm
    static readonly float width = 6 * scale; // 6 cm
    static readonly float height = 6 * scale; // 6 cm

    static float PtToCm(float pt) => pt / scale;



    public static void Generate(string filename)
    {
        yellowStroke.StrokeWidth = PtToCm(4); // 4pt
        redStroke.StrokeWidth = PtToCm(4); // 4pt

        using (var stream = new SKFileWStream($"{filename}.pdf"))
        using (var document = SKDocument.CreatePdf(stream))
        using (var canvas = document.BeginPage(width, height))
        {

            // translate first and then scale, don't reverse!
            canvas.Translate(width / 2, height / 2);
            canvas.Scale(scale);

            // draw a red circle
            for (int i = 0; i < 5; i++)
            {
                canvas.DrawCircle(0, 0, PtToCm(8) * (2 * i + 1), yellowStroke);
                canvas.DrawCircle(0, 0, PtToCm(8) * (2 * i + 2), redStroke);
            }

            document.EndPage();
        }
    }

    private static void Main()
    {
        string filename = nameof(ConcentricCircle);
        Generate(filename);

        // convert to PNG with ImageMagick
        using (Process p = new Process())
        {
            p.StartInfo.FileName = "magick";
            p.StartInfo.Arguments = $"convert -compose copy -bordercolor red -border 2x2 -density 200 -alpha remove {filename}.pdf {filename}.png";
            p.Start();
        }
    }
}

documentclass{standalone}
usepackage{tikz}

begin{document}
begin{tikzpicture}[mystyle/.style={circle,draw,fill=none,minimum size=20, line width = 8pt}]
  foreach x in {1,...,20}{
    pgfmathparse{isodd(x)}ifnumpgfmathresult=1defcurrcol{red!70}elsedefcurrcol{yellow!50}fi
    draw[line width=8pt,currcol] (0,0) circle (x cm);}
end{tikzpicture}
end{document}