There is only single longtable table in manuscript but why its number is displayed as “Table 2”?

\documentclass[fleqn,preprint,10pt]{elsarticle}
\usepackage{hyperref}
\usepackage{longtable}
\usepackage{caption}
\hypersetup{pdfstartview={XYZ null null 1.00}}%to get pdf zoomed to 100%
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}%for \mathfrak{g}
\begin{document}
\begin{longtable}{lll}
\hline\hline
% after \\: \hline or \cline{col1-col2} \cline{col3-col4} ...
\\ $N_a$ & $\text{Nor}_{\mathfrak{g}}\left(N_a\right)/N_a$ & Sub--algebras \\\\\hline\hline
$e_1$&$\{e_2, e_3, e_4, e_5\}$
\\\hline\hline\end{longtable}\captionof{table}{Classification of algebra $\mathfrak{g}=\,L_5\oplus L_4 \oplus L_3$.}\label{classi}
\end{document}


Longtable increases the table counter, with the intention that you use \caption within the environment, and then you have used \captionof{table} outside the environment.

Either change

\end{longtable}\captionof{table}{...}


to

\caption{...}\end{longtable}


or, for special circumstances where you really need no caption in the table, you could use

\end{longtable}\addtocounter{table}{-1}\captionof{table}{...}

• Where should I place caption then ? – IgotiT Jun 13 '16 at 7:01
• @IgotiT I added the code in an edit which I think crossed with your comment, is it clearer now? – David Carlisle Jun 13 '16 at 7:03
• Ok ok, I am sorry, I got it. – IgotiT Jun 13 '16 at 7:04