If a user has only shared data, and absolutely no owned data, we will transfer 0 files from source to destination for that particular user.
Unfortunately, we can't tell you how much data a user has until they have been transferred and you will still be billed on a per user basis. All user data is pooled for your use, so if a user in your organization owns no data then you'll have another 20GB to use with the others being migrated.
It's a good idea to do your homework! Some services, like Box, have a report you can generate from the Admin dashboard. In other cases, you may need to contact the related support staff in your source service to ascertain what users own what data. Information like this will also help you figure out your total migration costs.