I hope this isn't homework. Kinda sounds like it.
If you want to know how many more files there are than there were, have cron run this hourly - probably a better way, of course, but it may work.
Code:
find_cmd() { find DIR -type f | wc -l > file_number; }
if [ -f file_number ]; then
mv file_number file_number.old
find_cmd
echo $(($(cat file_number)-$(cat file_number.old)))
else
find_cmd
fi
If you just want to know how many files have been added, then this could work - though it'll just check timestamps - someone could 'touch' every file in the DIR and this would return every file.
Code:
if [ ! -f timestamp ]; then
touch timestamp
fi
find DIR -type f -newer timestamp | wc -l
touch timestamp
If these aren't right, at least they might give some ideas.