\documentclass{article}
\usepackage{tikz}

\usetikzlibrary{shapes}

\newcommand*\rect[1]{\tikz[baseline=(char.base)]{
\node[shape=rectangle, draw, inner sep=4pt, minimum width=2em,
fill=black, text=white, font=\bfseries, rounded corners = 4] (char) {#1};}}

\newcommand*\rectitem{%
\stepcounter{enumi}\item[\rect{\theenumi}]}
    \begin{document}
    Test:  \rect{1} \rect{20} \rect{500} baseline aligned.
\end{document}