On pourrait utiliser l'option `numwidth` :
\documentclass[a4paper,french,12pt]{report}
\usepackage{tocbasic}
\DeclareTOCStyleEntry[
beforeskip=.6em plus 1pt,% default is 1em plus 1pt
pagenumberformat=\textbf,
numwidth=3em
]{tocline}{chapter}
\renewcommand{\thechapter}{\Roman{chapter}}
\begin{document}
\tableofcontents
\chapter{Premier}
\chapter{Deux}
\chapter{Trois}
\chapter{Quatre}
\chapter{Cinq}
\chapter{Six}
\chapter{Sept}
\chapter{Huit}
\chapter{Neuf}
\chapter{Dix}
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screenshot_2024-01-11_at_16.51.22.png
