I am not sure I understand the question, but maybe you are looking for this. You can define a new tag form that adds multiple equation number, where the number can be set with \MultiTag.

\documentclass[a4paper,12pt]{article} \usepackage[spanish]{babel} \usepackage[latin1]{inputenc} \usepackage{mathtools} \newcounter{iloop} \newcounter{imax} \newtagform{Ignacio}{(}{\setcounter{iloop}{0}\loop% \stepcounter{iloop}% \ifnum\value{iloop}<\value{imax}% \stepcounter{equation},% \number\value{equation}\repeat)} \newcommand{\MultiTag}[1]{\setcounter{imax}{#1}} \begin{document} \usetagform{Ignacio}\MultiTag{3}% Einstein \begin{equation} E=mc^2 \end{equation} Photon \begin{equation} E=h\nu \end{equation} Thermo\MultiTag{2} \begin{equation} E=TS \end{equation} \end{document} 

This allows you to add multiple equation numbers via a command \multitag.

\documentclass[a4paper,12pt]{article} \usepackage[spanish]{babel} \usepackage[latin1]{inputenc} \usepackage{mathtools} \newcounter{iloop} \newcounter{imax} \newcommand{\MultiNum}{\edef\temp{}% \setcounter{iloop}{0}\loop% \stepcounter{iloop}% \ifnum\value{iloop}<\value{imax}% \edef\temp{\temp\number\value{equation}),(}\refstepcounter{equation}% \repeat% \edef\temp{\temp\number\value{equation}}% \refstepcounter{equation}} \newcommand\multitag[1][1]{\setcounter{imax}{#1}% \MultiNum% \tag{\temp}} \begin{document} Einstein  \begin{equation}\label{eq:Einstein} E=mc^2 \multitag[3] \end{equation} Photon \begin{equation}\label{eq:Photon} E=h\nu \multitag[2] \end{equation} Thermo \begin{equation}  E=TS \end{equation} Wrong \begin{equation}\label{eq:Wrong} E=mc^3  \end{equation} \eqref{eq:Wrong} is wrong, \eqref{eq:Einstein} is correct, and so is \eqref{eq:Photon}. \end{document}