How can I number sections below subsection in LaTeX?

You have to insert setcounter{secnumdepth}{5} to your document preamble and then the command paragraph{title} is the equivalent of what you mean by subsubsubsection{title}.

But in my opinion is not a good idea to have numbered paragraphs nor numbered subsubsections. It makes the text much less readable.

documentclass{report}
setcounter{secnumdepth}{5} % seting level of numbering (default for "report" is 3). With ''-1'' you have non number also for chapters
 %setcounter{tocdepth}{5} % if you want all the levels in your table of contents
begin{document}
chapter{chapter title (depth 0)}
section{section title (depth 1)}
subsection{subsection title (depth 2)}
subsubsection{subsubsection title (depth 3)}
paragraph{paragraph title (depth 4)}
subparagraph{Subparagraph title (depth 5)} % last existing level
end{document}