This comes from an old message to the mailing list:
http://osdir.com/ml/gdb.patches/2002-11/msg00043.html

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The following patch adds a new user-visible variable to GDB,
'max-completion-results'.

It is used by the completion functions in symtab.c to limit the maximum number
of results returned by the completion function, and defaults to 'unlimited'. I
find it particuarly useful when using GDB under emacs, to prevent situations
where hitting <tab> at the wrong time can lead to hundreds of thousands of
results having to be sorted by complete_command. 
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Patch by Klee Dienes. Klee sent me in November 2008 an updated version of the
patch, which I will attach here. Some more adaptation may be needed.