align columns in tabular

Question

In the following tabular, When I get to item 10, the extra digit causes the word item to be pushed along.

How do I make all the words 'item' aligned?

MWE:

\documentclass[10pt]{article}
\usepackage{booktabs,array}
\begin{document}
\newcounter{rowcounter}
\setcounter{rowcounter}{1}
\begin{tabular}{@{\arabic{rowcounter}\stepcounter{rowcounter}}@{\hspace{1cm}}ll}
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
\end{tabular}%
\end{document}

Answer 1

As @Zarko has suggested in a comment, if you don't want the implicit column contents (and hence the contents of the first "real" column) to be staggered, make the column explicit and give it type r.

\documentclass{article}
\usepackage{array}
\newcounter{rowcnt}
\begin{document}
\begin{tabular}{@{} >{\stepcounter{rowcnt}\therowcnt}r l }
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
&Item\\
\end{tabular}
\end{document}

Answer 2

I propose here an improvement to your code with eqparbox, and a solution with blkarray which defines a referable row number via a new column type,\BAenum:

\documentclass[10pt]{article}
\usepackage{booktabs, blkarray}
\usepackage{eqparbox}

\begin{document}

\newcounter{rowcounter}
\setcounter{rowcounter}{1}
\begin{tabular}{@{\eqmakebox[R][r]{\arabic{rowcounter}}\stepcounter{rowcounter}}@{\hspace{1cm}}ll}
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
\end{tabular}%
\vskip 4ex
\noindent
\begin{blockarray}{\BAenum!{\hspace{1cm}}ll}
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
Item\\
 \label{row:ten} Item\\
Item\\
Item\\
Item\\
\end{blockarray}%

From row \ref{row:ten}, we can see the row numbers are right-aligned.

\end{document}