How to insert a number between columns in latex table?

Probably something like the following?

\documentclass[]{article}
\usepackage{calc}
\usepackage{array}
\setlength{\tabcolsep}{3pt}
\newcommand{\mc}[1]{\multicolumn{2}{>{\centering\arraybackslash}p{\widthof{0.000}}}{#1}}
\begin{document}

\begin{table}[t!] 
\caption{Moderating effect} 
\bigskip
\centering

\begin{tabular}{l*{10}{p{\widthof{00}}}} \hline \hline 
Numbers    & \mc{1} & \mc{2} & \mc{3} & \mc{4} & \mc{5}\\
Mean       & \mc{2.658} & \mc{1.923}& \mc{1.625}&  \mc{1.480}&  \mc{1.020}\\
Difference & & \mc{0.736} & \mc{0.298} & \mc{0.144} &\mc{0.461}\\

\end{tabular}
\end{table}
\end{document}

Alternatively and as suggested by Sveinung one can also use the following definition of \mc:

\newcommand{\mc}[1]{\multicolumn{2}{wc{\widthof{0.000}}}{#1}}

Add more columns. I also introduced a small amount of negative space.

\documentclass{article}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{lc*{8}{@{\hspace{-4pt}}c}}
\toprule
Numbers    &   1   &&   2   &&   3   &&   4   &&   5   \\
\midrule
Mean       & 2.658 && 1.923 && 1.625 && 1.480 && 1.020 \\
Difference &&     0.736 && 0.298 && 0.144 && 0.461 \\
\bottomrule
\end{tabular}

\end{document}

It is not very clear what you like to obtain, It can be understand as follows:

\documentclass{article}
\usepackage{booktabs, threeparttable}
\usepackage{siunitx}

\begin{document}
    \begin{table}[ht]
\caption{Moderating effect}
\label{tab:diferences}
\centering
\footnotesize
\begin{threeparttable}
\begin{tabular}{l*{9}{S[table-format=1.3]} } 
    \toprule
Numbers & 1     &{diff} & 2     &{diff} & 3     &{diff} & 4     &{diff} & 5         \\
Mean    & 2.658 & 0.736 & 1.923 & 0.298 & 1.625 & 0.144 & 1.480 & 0.461 & 1.020     \\   
    \bottomrule
\end{tabular}
\begin{tablenotes}\footnotesize
\item   fdfdfd
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}

or

\documentclass{article}
\usepackage{booktabs, threeparttable}
\usepackage{siunitx}

\begin{document}
    \begin{table}[ht]
\caption{Moderating effect}
\label{tab:diferences}
\centering
\footnotesize
\begin{threeparttable}
\begin{tabular}{l*{9}{S[table-format=1.3]} }
    \toprule
Numbers     & 1     &{diff} & 2     & {diff} & 3     &{diff} & 4     & {diff} & 5       \\
Mean        & 2.658 &       & 1.923 &        & 1.625 &       & 1.480 &        & 1.020   \\
    \midrule
Differences &       & 0.736 &       & 0.298  &       & 0.144 &       & 0.461  &         \\
    \bottomrule
\end{tabular}
begin{tablenotes}\footnotesize
\item   fdfdfd
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}

If these are not a case, please provide a sketch, which show what you like to have.