Changing distance between chapter number and chapter title in the table of contents

The tocstyle package (included in KOMA-script) by default automatically calculates the needed indentation for chapter and other sectioning numbers in the ToC. (It needs several LaTeX runs to do so.)

\documentclass{report}

\usepackage{tocstyle}
\usetocstyle{standard}

\begin{document}

\tableofcontents

\setcounter{chapter}{5000}

\chapter{foo}

\end{document}

You could use the titletoc package (which is a companion for titlesec).

From the titlesec/titletoc manual page 18:

[Edit: titlesec/titletoc manual page 17:]

\titlecontents{<section>}[<left>]{<above>}
              {<before with label>}{<before without label>}
              {<filler and page>}[<after>]

[See also The titlesec, titleps, and titletoc Packages (2016 Mar 21) pp.14, 16 (which provides a similar \titlecontents command option).]

Example:

\documentclass[a4paper]{book}
\usepackage[english]{babel}
\usepackage{titletoc}
\usepackage{blindtext}
\titlecontents{chapter}[1.5em]{\addvspace{1pc}\bfseries}{\contentslabel{5em}}{}
    {\titlerule*[0.3pc]{.}\contentspage}
\begin{document}
\tableofcontents
\blinddocument
\end{document}

If you are satisfied with the normal typesetting of the book class, you can do with "minimal" effort:

\usepackage{etoolbox}
\makeatletter
\patchcmd{\l@chapter}{1.5em}{2em}{}{}
\makeatother

You may also have a look at the memoir class, which must be considered for a 100+ chapter project; it has tools for customizing every aspect of a book, for example

\setlength{\cftchapternumwidth}{2em}

would suffice.