How to change the page number for the scrartcl class to # of ## pages on the first page

Please see the following MWE. Important code is marked with <=======.

\documentclass[a4paper, oneside, 12pt]{scrartcl}

\usepackage[%
  footsepline=0.25pt, headsepline=0.25pt,
  % automark places section title in header. Also enables placement in footer.
  automark
]{scrlayer-scrpage} % <=================================================

\usepackage[a4paper,
  vmargin=2cm, hmargin=2cm, % page margins
  includehead, includefoot, % Margins calculated include header and footer
  footskip=2em]
{geometry}

\usepackage{blindtext}
\usepackage{lastpage} % <===============================================

\ihead{\rightmark}
\chead{}
\ohead{\leftmark}
\ifoot{}
\cfoot{\thepage\ of \pageref{LastPage}} % <=============================
\ofoot{}
\pagestyle{scrheadings}

\begin{document}
\section{Testing}
\blinddocument
\end{document}

It uses scrlayer-scrpage and lastpage with the following result:

With a KOMA-Script class this can be done without a package for header and footer. You only have to redefine the KOMA-Script command \pagemark:

\documentclass[12pt]{scrartcl}
\usepackage{blindtext}
\usepackage{lastpage}
\renewcommand\pagemark{{\usekomafont{pagenumber}\thepage\ of \pageref{LastPage}}}

\author{Author}
\title{Title}
\begin{document}
\maketitle
\section{Testing}
\blinddocument
\end{document}

It is also possible to add lines below the header and above the footer. If you set pagestyle headings you will get the sections in header.

\documentclass[12pt,
  headsepline,footsepline% <- added
]{scrartcl}
\usepackage{blindtext}
\usepackage{lastpage}
\renewcommand\pagemark{{\usekomafont{pagenumber}\thepage\ of \pageref{LastPage}}}
\pagestyle{headings}% <- added

\author{Author}
\title{Title}
\begin{document}
\maketitle
\section{Testing}
\blinddocument
\end{document}

---

If you want to customize header and footer, you can use package scrlayer-scrpage which is part of the KOMA-Script bundle:

\documentclass[12pt]{scrartcl}
\usepackage{blindtext}
\usepackage{lastpage}
\renewcommand\pagemark{{\usekomafont{pagenumber}\thepage\ of \pageref{LastPage}}}
\usepackage{scrlayer-scrpage}% sets pagestyle scrheadings automatically
\clearpairofpagestyles
\ohead{\headmark}
\ofoot*{\pagemark}

\author{Author}
\title{Title}
\begin{document}
\maketitle
\section{Testing}
\blinddocument
\end{document}

The starred version of \ofoot sets the entry for both scrheadings and plain.