As a rule of thumb all manager level and above should be given an email account; supervisory level who needs to communicate with customer or supplier should be given one as well. Those staff who need an email must apply base on their job function and requirement and approved by their respective managers before assigning one to them. The rest of the users can shared or do not need one as they can be informed through notice board or word of mouth not necessary an email account (cost purpose).
Basically 90 emails account is a bit much but really it depends on your requirement following the above rules. Anyway if you can keep it around 50 – 60 it should be quite optimum I think for APT, but as I am not involved in APT operations this is just a ballpark figure; I might be wrong.