How to perform simple arithmetic with page number?

There are a few options.

If you want to globally change the page number, you can use \addtocounter or \setcounter, as recommended by Thorsten Donig.

If you want to use the result in a length or counter, as in \setcounter, \addtocounter, \setlength, or \addtolength, you can use the calc package. For example, one less than the page number is \setcounter{pagelessone}{\thepage-1}, where pagelessone is some counter you've already defined.

If you want to do something more complicated, you can use the eTeX primitives \dimexpr and \numexpr. For example, one less than the page number is \numexpr\thepage-1\relax. For more information, see section 3.5 of the eTeX documentation.