The documentation (https://www.mediawiki.org/wiki/Extension:SyntaxHighlight_GeSHi) claims that the line attribute can be empty (default=fancy) or explicitly set to fancy or default. Fancy means every nth line number (default n=5) is bolded.
Firstly this is a lie, the code only detects the presence of 'line' and sets fancy mode regardless of the value.
Secondly there is no way to customise the value of n.