It would be conveninet for end users if the above is included as a macro in AMC like:
\AMCSamesize{16}.

I tried this solution and find three errors in the code. Therefore, I'd like to share my solution in order to help others to have every realization with the same number of pages.

The errors are:
1. The last ifthenelse-element lacks a "}{}"
2. The second package needed is called "pgffor"
3. There is a missing "}" at the end of the definition of \blankbookpage

Therefore, a working solution for a minimum of 16 pages would be:

1. Calculate the number of pages by putting \AMCqlabel{paperends} at the end of the latex file (before the } of \onecopy{1}{ and \end{document})

\AMCqlabel{paperends} 
}
\end{document}

2. For a document of a minimum of 16 pages: put the following code before the \AMCqlabel{paperends}

\ifthenelse{\equal{\AMCpageref{paperends}}{11}}{\forntimes{5}{\blankbookpage}}{}
\ifthenelse{\equal{\AMCpageref{paperends}}{12}}{\forntimes{4}{\blankbookpage}}{}
\ifthenelse{\equal{\AMCpageref{paperends}}{13}}{\forntimes{3}{\blankbookpage}}{}
\ifthenelse{\equal{\AMCpageref{paperends}}{14}}{\forntimes{2}{\blankbookpage}}{}
\ifthenelse{\equal{\AMCpageref{paperends}}{15}}{\forntimes{1}{\blankbookpage}}{}

3. To define the macros "\forntimes" and "\blankbookpages" put the following code in the preamble of the document (before \begin{document}). In order to do so, the packages "forntimes" and "pgffor" are needed

\usepackage{ifthen} 
\usepackage{pgffor} 

\newcommand{\forntimes}[2]{
    \foreach \n in {1,...,#1}{#2}}
\newcommand{\blankbookpage}{\newpage \tiny \underline{\textsc{Space For Rough Work}}}

It would be conveninet for end users if the above is included as a macro in AMC

From version hg:4c043e0885fd, you can use \AMCaddpagesto{16} at the end of the student's test description.