By default, chapters in LaTeX are numbered using Arabic numerals (1, 2, 3, ...). However, it's possible to change the format of chapter numbers to Roman numerals (I, II, III, ...) as well.
To do this, you can use the following command:
\renewcommand\thechapter{\Roman{chapter}}
When using Roman numerals for chapter numbering in LaTeX, it's important to note that there is no Chapter 0. If you want to include a Chapter 0 for an introduction or similar section, you must create it before changing to Roman numerals. Otherwise, you'll end up with a chapter without any numbering.