There's no way to easily customize the number of issues
to display at once by default.

The current, hard-coded value of 50 is used in three
places: 

* cgi/templating.py, which can probably be easily
updated to use the tracker's configuration,

* templates/classic/html/issue.search, and 
* templates/classic_htmlbase.py, which I didn't really
examine.

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The htmlbase occurance will be mirroring the issue.search one - 
you may ignore it.

The one in cgi/templating is a fallback value for when nothing 
else supplies one.

To control the page size, add a URL variable ":pagesize=100" or 
whatever value you like.

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I don't count "add a parameter to every URL" as
"customizable", unless you mean there's some central place I
can do this for all URLs that call up issue.index. Is there one?