will give you just the unique numbers, most likely a sorted list from 0 to the maximum of labels someone has used.
gam user us...@yourdomain.com print messages max_to_print 2 showlabels | cut -d',' -f12 | while read LINE; do echo $LINE | wc -w; done | sort -u | tail -1
will give you the maximum amount of labels used on a single message in the sample.
This is a bit rough but it will work. If you want, you could loop it through a number of users to get a better view. It will take more time with more users, obviously:
gam print users | tail +2 | while read USER; do
gam user ${USER} print messages showlabels | cut -d',' -f12 | while read LINE; do echo $LINE | wc -w; done | sort -u | tail -1
done
This will produce a list of maximum per message label counts with one entry for every user.