A serious theological reply would be that this question makes no sense, because demons, like angels, are spirits and consequently do not have spatial extension, so the number that can be 'in' a person has no limit.
A frivolous suggestion: look up the cardinal's curse in the poem 'The Jackdaw of Rheims', assume the arraival of one demon per activity or part of body cursed, and add ...
Jacqueline Simpson

--- On Sun, 26/9/10, James John Bell <[log in to unmask]> wrote:

From: James John Bell <[log in to unmask]>
Subject: [ACADEMIC-STUDY-MAGIC] How many demons can fit in one person?
To: [log in to unmask]
Date: Sunday, 26 September, 2010, 4:42

Hey folks,

Working on an article for Halloween release, and trying to track down the
definitive number of demons that have ever inhabited a single person at one
time, could be mythic or historic.

Online it appears that the number from bible scholars is seven, but
contemporary fundamentalist Christians say "as many as 6000 demons":

Anyone have a source on this number? Seems like a lot.