LaTeX.org

\documentclass{article}
\usepackage{amsmath}
\setcounter{MaxMatrixCols}{11}
\begin{document}
\[
A = \begin{pmatrix}
a_{11} & a_{12} & a_{13} & a_{14} & a_{15} & a_{16} & a_{17} & a_{18} & a_{19} & a_{10} & a_{11} \\
a_{21} & a_{22} & a_{23} & a_{24} & a_{25} & a_{26} & a_{27} & a_{28} & a_{29} & a_{20} & a_{21} 
\end{pmatrix}
\]
\end{document}

Internally, [latex]amsmath[/latex] uses an [latex]array[/latex] environment with a number of columns set to [latex]MaxMatrixCols[/latex]. That is a counter, intially set to 10. That's usually enough, but in case we need bigger matrices, we can modify the counter, also globally, by [latex]\setcounter{MaxMatrixCols}{<value>}[/latex]. Here, we set the [latex]MaxMatrixCols[/latex] value to 11, and now it compiles without error:

[code]\documentclass{article}
\usepackage{amsmath}
\setcounter{MaxMatrixCols}{11}
\begin{document}
\[
A = \begin{pmatrix}
a_{11} & a_{12} & a_{13} & a_{14} & a_{15} & a_{16} & a_{17} & a_{18} & a_{19} & a_{10} & a_{11} \\
a_{21} & a_{22} & a_{23} & a_{24} & a_{25} & a_{26} & a_{27} & a_{28} & a_{29} & a_{20} & a_{21}
\end{pmatrix}
\]
\end{document}[/code]

We get:

[attachment=0]matrix.png[/attachment]

Of course, we can also set it to a higher value. Btw. we had that question here earlier, in the topics [url=https://latex.org/forum/viewtopic.php?f=44&t=22804]Matrix Alignment Error[/url] and [url=https://latex.org/forum/viewtopic.php?f=46&t=5996]Problem with creating a matrix[/url].

Stefan

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
A = \begin{pmatrix}
a_{11} & a_{12} & a_{13} & a_{14} & a_{15} & a_{16} & a_{17} & a_{18} & a_{19} & a_{10} & a_{11} \\
a_{21} & a_{22} & a_{23} & a_{24} & a_{25} & a_{26} & a_{27} & a_{28} & a_{29} & a_{20} & a_{21} 
\end{pmatrix}
\]
\end{document}

I used the code for Figure 9.34 in [i]Chapter 9, Writing Math Formulas[/i], to write a matrix with the [latex]amsmath[/latex] package.

With a matrix of 11 or more columns I get an error. Here is the example:

[code]\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
A = \begin{pmatrix}
a_{11} & a_{12} & a_{13} & a_{14} & a_{15} & a_{16} & a_{17} & a_{18} & a_{19} & a_{10} & a_{11} \\
a_{21} & a_{22} & a_{23} & a_{24} & a_{25} & a_{26} & a_{27} & a_{28} & a_{29} & a_{20} & a_{21}
\end{pmatrix}
\]
\end{document}[/code]

I get the error:

[quote]! Extra alignment tab has been changed to \cr.
<recently read> \endtemplate

l.6 ..._{16} & a_{17} & a_{18} & a_{19} & a_{10} &
a_{11} \\[/quote]

Is there a limit of 10? How can I have more?