Avec Ça marche avec `\mil{4}{6}` :
\documentclass{standalone}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage[svgnames]{xcolor}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{array}
\usepackage{tabularray}
\usepackage[french]{babel}
\usepackage{tkz-euclide}
\usepackage{kpfonts}
\usetikzlibrary{intersections,positioning,backgrounds,fit,calc}
\usetikzlibrary{through}
\usetikzlibrary{positioning, calc}
\newcommand\mil[2]{
\begin{scope}
\draw[very thin,LightBlue,step=0.1] (0,0) grid (#1,#2);
\draw[thin,LightBlue,step=0.5] (0,0) grid (#1,#2);
\draw[thick,DarkBlue] (0,0) grid (#1,#2);
\end{scope}
}
\begin{document}
\begin{tikzpicture}
\mil{4}{6}
\end{tikzpicture}
\end{document}
