Numbering appendices by letter instead of number

If you really need the appendices to be subsections you can change numbering of subsections using \renewcommand{\thesubsection}{\Alph{subsection}}:

\documentclass[11pt, a4paper, twoside]{article}
\usepackage{geometry}
\usepackage{lipsum}

\begin{document}
\tableofcontents

\section{First section}
\subsection{A subsection}
\lipsum[1]

\appendix
\section*{Appendices}
\addcontentsline{toc}{section}{Appendices}
\renewcommand{\thesubsection}{\Alph{subsection}}

\subsection{Appendix Subsection}
\subsection{Another appendix Subsection}
\end{document}

Use \section*{Appendices}and add this to the toc with \addcontentsline. Then simply use \section for each appendix.

\documentclass{article}
\begin{document}
\tableofcontents
\section{First section}
\subsection{A subsection}
\appendix
\addcontentsline{toc}{section}{Appendices}
\section*{Appendices}
\section{First appendix}
\section{Second appendix}
\end{document}

I find it easier to simply use:

\appendix
\renewcommand{\thechapter}{A}

and just change it for each appendix:)