This question has already been asked (How to change certain pages into landscape/portrait mode), but as a comment says, the given solution puts the page number on one of the side at with a 90° rotation:
\usepackage{pdflscape}
...
\begin{landscape}
...
\end{landscape}
I would think this might be logical if you actually print the stuff, since then you would stack it in with the portrait pages and this way you will have the page number on the same physical location, but it is quite confusing when viewing on the monitor. Is there a way to place the page number at the bottom of the page?