\setuppagenumbering [alternative=doublesided,location=]
\setupbodyfont[ss,150pt]
\startbuffer[DemoPage]
    \hfill
    \midaligned{\bold{\pagenumber}}
    \hfill
\stopbuffer


\definepapersize[Portrait] [A4]
\definepapersize[Landscape][A4,landscape]


\starttext
\setuppapersize[Portrait]
\getbuffer[DemoPage] \page
\getbuffer[DemoPage] \page

\page\adaptpapersize[Landscape]
\getbuffer[DemoPage] \page
\getbuffer[DemoPage] \page
\stoptext