documentclass{minimal}
usepackage{pgfplots}
usepgfplotslibrary{statistics}

begin{document}

   begin{tikzpicture}
   begin{axis}[y=1cm]
      foreach n in {1,...,8}
      {
      addplot+ [boxplot,draw=black,solid,fill=white] table [row sep=\,y index=0]
         {
         data\
         1\ 2\ 1\ 5\ 20\ 10\
         7\ 10\ 9\ 8\ 9\ 9\
         };
      }
   end{axis}
   end{tikzpicture}
end{document}

% used PGFPlots v1.16
documentclass[border=5pt]{standalone}
usepackage{pgfplots}
    usepgfplotslibrary{statistics}
    pgfplotsset{
        compat=1.3,
        % create a custom style for the boxplots
        my boxplot style/.style={
            boxplot,
            draw=black,
            solid,
            fill=white,
            % -------------------------------------------------------------
            % add your desired mark symbol and mark style here
            mark=*,
            every mark/.append style={
                fill=gray,
            },
            % -------------------------------------------------------------
        },
    }
begin{document}
begin{tikzpicture}
   begin{axis}
        foreach n in {1,...,8} {
            addplot+ [
                % apply the custom style
                my boxplot style,
            ] table [row sep=\,y index=0] {
                data\
                1\ 2\ 1\ 5\ 20\ 10\
                7\ 10\ 9\ 8\ 9\ 9\
            };
        }
   end{axis}
end{tikzpicture}
end{document}