To have an automated and updated counter of your pages between two labels you can use the following commands/packages:
Place in main document:
\usepackage{refcount}
\newcommand{\pagedifference}[2]{%
\COMMENT{ \number\numexpr\getpagerefnumber{#2}-\getpagerefnumber{#1}\relax}}
To print a page difference:
\pagedifference{chap:start}{chap:end}