How to split columns into 2

Question

I want the first row to stay normal and then every row after that should be split into 2 columns. I've done it on Microsoft word to show exactly what I want. I tried used multicolumn but couldn't get it to work correctly.

\begin{center}
\begin{tabular}{ |c|c|c|c|c|c|c|c| } 
    \hline
    B    & B 1 & B 2 & B3 & B4 & B5 & B6 & B 7 \\
    \hline
    1   & 12 & 12 & 12 & 12 &12  &12& 12\\ 
    \hline
    2   & 12& 12& 12& 12&12& 12& 12\\ 
    \hline
    3   & & & & & & & \\ 
    \hline
    4   & & & & & & &\\ 
    \hline
    5   & & & & & & &\\ 
    \hline
    6   & & & & & & & \\ 
    \hline
    7   & & & & & & &\\ 
    \hline
\end{tabular}
\end{center}

Answer 1

• your table code fragment doesn't reproduce showed table. for this it ha defined one column less as needed (you have defined only 8 columns, but showed table has 9)
• for help to people who like to help you, y\documentclass{...} and ending with `\end{document}, which show your problem. it should have in preamble loaded all to your problem relevant packages (if any needed)
• as i mentioned in comment, cells cant be split, but can be merged. so solution in general in your case is use command \multicolumn{2}{c}{...} which merge tvo columns and cell's content align in its middle.
• without knowing content of your table your code snippet can be converted into mwe as follows:

\documentclass{article}

\begin{document}
    \begin{center}
\begin{tabular}{| *{9}{c|} }
    \hline
B    & \multicolumn{2}{c|}{x}
            & \multicolumn{2}{c|}{z}
                    & \multicolumn{2}{c|}{w}
                            & \multicolumn{2}{c|}{t}                \\
    \hline
1   &   12  &   12  &   12  &   12  &   12  &   12  &   12  &   12  \\
    \hline
2   &   12  &   12  &   12  &   12  &   12  &   12  &   12  &   12  \\
    \hline
3   &       &       &       &       &       &       &       &       \\
    \hline
\end{tabular}
    \end{center}
\end{document}

• however, your table can be written on many diferent way. one of them is usebooktabs packages for horizontal rules where needed, omit vertical riles and for example use modified X columns from package `tabularx:

\documentclass{article}
\usepackage{booktabs, tabularx}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\begin{document}
    \begin{center}
\begin{tabularx}{\linewidth}{ c *{8}{C} }
    \toprule
B    & \multicolumn{2}{c}{x}
            & \multicolumn{2}{c}{z}
                    & \multicolumn{2}{c}{w}
                            & \multicolumn{2}{c}{t}                \\
    \midrule
1   &   12  &   12  &   12  &   12  &   12  &   12  &   12  &   12  \\
2   &   12  &   12  &   12  &   12  &   12  &   12  &   12  &   12  \\
3   &       &       &       &       &       &       &       &       \\
    \bottomrule
\end{tabularx}
    \end{center}
\end{document}

Answer 2

\documentclass[12pt,a4paper]{article}
\usepackage[left=.45in,right=.45in,top=.6in,bottom=.6in,headheight=14.5pt]{geometry}
\usepackage{array,multirow,textcomp}
\usepackage[T1]{fontenc}

\newcolumntype{L}[1]{>{\raggedright\let\newline\\\arraybackslash\hspace{0pt}}m{#1}}
%\newcolumntype{R}[1]{>{\raggedleft\let\newline\\\arraybackslash\hspace{0pt}}m{#1}}
%\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\newcolumntype{C}[1]{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}m{#1}}

 %    \setlength\extrarowheight{32pt}
\begin{document}

\centering
\begin{tabular}{|c|C{1cm}|C{1cm}|C{1cm}|C{1cm}|C{1cm}|C{1cm}|C{1cm}|C{1cm}|C{1cm}}
    \hline
 x & \multicolumn{2}{c|}{y}& \multicolumn{2}{c|}{z} & \multicolumn{2}{c|}{w}& \multicolumn{2}{c|}{t}\\ \hline
 1 &&&&&&&&\\ \hline
 2 &&&&&&&&\\ \hline
 3  &&&&&&&&\\ \hline
 4  &&&&&&&&\\ \hline
 5  &&&&&&&&\\ \hline
 6  &&&&&&&&\\   \hline     
 7  &&&&&&&&\\ \hline


\end{tabular}

 \end{document} 

Answer 3

I recommend this website. It's really easy and fast to use. The table in my answer is generated by the website.

https://www.tablesgenerator.com/

% Please add the following required packages to your document    
% \usepackage{multirow}
     
\begin{table}[]
\begin{tabular}{cc}
\multicolumn{2}{c}{Count} \\
A           & 25          \\
B           & 47         
\end{tabular}
\end{table}