How to place page number in top right corner of title page using scartcl class

You could use

fancypagestyle{plain}{}

This makes page style plain the same as fancy. The second (empty) parameter can be used to add additional definitions if you want plain to be a little different from fancy. So if you don't want the running header on the left, and not the line, use

fancypagestyle{plain}{fancyhead[R]{}renewcommand{headrulewidth}{0pt}}

With a KOMA-Script class it is recommended to use package scrlayer-scrpage for header and footer.

Example:

documentclass{scrartcl}
usepackage{blindtext}% only for dummy text

usepackage[automark,headsepline,markcase=upper]{scrlayer-scrpage}% sets page style scrheadings automatically
clearpairofpagestyles
ihead{rightmark}
ohead*{pagemark}% short version of ohead[pagemark]{pagemark}
addtokomafont{pagehead}{normalfont}

begin{document}
title{Title}
author{Author}

maketitle
blinddocument
end{document}