Page (and other) numbers

To change the page number of the current page to 25, use \setcounter{page}{25} somewhere in the middle of the page.
If a couple of pages come from another source than the LaTeX file, you can increment the page number by 2 as
\addtocounter{page}{2}
Negative increments are allowed.
To get page numbers to appear as "1 of 3", "2 of 3", "3 of 3", add the beginning of the document use
\renewcommand{\thepage}{\arabic{page} of 3}
If you want roman numerals, use \roman instead of \arabic.

To change figure, table, chapter, section, footnote, ... numbers, you can replace "page" in the above commands by "figure", "table", "chapter", "section", "footnote", ... For these you use one unit less than the desired number just before the relevant figure, table, chapter, section, ....

However, sectional units are somewhat tricky. If three chapters of your book are to come from an external source, you can use \addtocounter{chapter}{3} and \setcounter{page}{...} before the following chapter in your document. But what if you want to resume inside the third external chapter with say section 2? There will be difficulties with \label commands and section numbers. Try using \addtocounter{chapter}{2}, \refstepcounter{chapter}, \refstepcounter{section}.