My answer to How to set the number of the first citation? can also be applied if one uses bibunits
:
\documentclass{article}
\usepackage[numbers]{natbib}
\usepackage{bibunits}
\usepackage{etoolbox}
\makeatletter
\newcommand*{\newbibstartnumber}[1]{%
\apptocmd{\thebibliography}{%
\global\c@NAT@ctr #1\relax
\addtocounter{NAT@ctr}{-1}%
}{}{}%
}
\makeatother
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@misc{A01,
author = {Author, A.},
year = {2001},
title = {Alpha},
}
@misc{B02,
author = {Buthor, B.},
year = {2002},
title = {Bravo},
}
@misc{C03,
author = {Cuthor, C.},
year = {2003},
title = {Charlie},
}
@misc{D04,
author = {Duthor, D.},
year = {2004},
title = {Delta},
}
\end{filecontents}
\usepackage{lipsum}
\begin{document}
\begin{bibunit}[plainnat]
\nocite{A01,B02}
\putbib[\jobname]
\end{bibunit}
\newbibstartnumber{15}
\begin{bibunit}[plainnat]
\nocite{C03,D04}
\putbib[\jobname]
\end{bibunit}
\end{document}
