I've finally gotten the time to look at this again.
I think I've found it.
This is what I did:
* Looked at some code in the location Barry suggested, for our installation it was F:\SAS\Config\APM93\SASEnvironment\SASCode\Jobs\usageReports\artifactTopTen.sas
* Observe there is a call to %artifactReportSubSet; near the top of the code, it was two lines above the reference to &numWeeks.
* Looked in the Macro directory, for us it was F:\SAS\Config\APM93\SASEnvironment\SASMacro\artifactreportsubset.sas, chose the job which looked to have the macro in it (pretty well named I'd say)
* Find that the contents has all the magic figures I'm interested in
I'm more comfortable changing the figure in one location, rather than hard coding it in individual jobs.
Hopefully if anyone else has this issue, this might help.
I think I was not following Barry very well because it looks as though we've on 9.3 and he was on 9.2.