single landscape page with page number at the bottom?

easiest way would be to use

\begin{landscape}
\thispagestyle{empty}

....
\vfill
\raisebox{-??}{\makebox[\linewidth]{\thepage}}
\end{landscape}

adjusting the raisebox amount to position it "by eye" into the space normally used for the left margin.

Note \thepage here is "safe" as landscape forces a pagebreak with \clearpage so you do not have the usual problem about references to page numbers not being known within the document.

The hard way to do it would be to use:

\documentclass{article}
\usepackage{pdflscape}
\usepackage{everypage}
\usepackage{lipsum}

\newcommand{\Lpagenumber}{\ifdim\textwidth=\linewidth\else\bgroup
  \dimendef\margin=0 %use \margin instead of \dimen0
  \ifodd\value{page}\margin=\oddsidemargin
  \else\margin=\evensidemargin
  \fi
  \raisebox{\dimexpr -\topmargin-\headheight-\headsep-0.5\linewidth}[0pt][0pt]{%
    \rlap{\hspace{\dimexpr \margin+\textheight+\footskip}%
    \llap{\rotatebox{90}{\thepage}}}}%
\egroup\fi}
\AddEverypageHook{\Lpagenumber}%

\begin{document}
\begin{landscape}
\pagestyle{empty}%
\lipsum[1]
\newpage
\noindent\rule{\linewidth}{\textheight}% fill text area
\end{landscape}
\pagestyle{plain}%
\lipsum[2]
\end{document}

Then again, this will also work for multiple pages.

Note that the page hook runs after the page of formatted but before the page number is incremented. At this point the "cursor" is located 1in below and 1in to the right of the upper left corner, and must not be moved.

For isolated pages needing the "landscaped page number" at the bottom, I like the idea from David Carlisle's approach, but couldn't get it to work. (The \raisebox didn't seem to have an effect for me, seems the shifts are absorbed by the \vfill.) Here's what I defined to get that idea to work:

\def\fillandplacepagenumber{%
 \par\pagestyle{empty}%
 \vbox to 0pt{\vss}\vfill
 \vbox to 0pt{\baselineskip0pt
   \hbox to\linewidth{\hss}%
   \baselineskip\footskip
   \hbox to\linewidth{%
     \hfil\thepage\hfil}\vss}}

Using that definition, one can now place a "landscaped page number" as follows:

% Preamble needs: \usepackage{pdflscape}
\begin{landscape}
  Landscape content.
  \fillandplacepagenumber
\end{landscape}

For several landscape pages in a row, one can just repeat separate copies of the block shown above, or the following also works:

% Preamble needs: \usepackage{pdflscape}
\begin{landscape}
  Landscape content.
  \fillandplacepagenumber
\newpage
  More landscape content.
  \fillandplacepagenumber
\end{landscape}

Possibly an easy fix to David Carlisle's \raisebox approach exists and thus would be a simpler (and more LaTeX-like) solution than the messy \fillandplacepagenumber shown here.