A few days ago I tried to make new wizards on my account, well when I had made my sixth wizard it wouldn't let me make another. I think you should be able to make seven wizards, one for every school. If you had one wizard for every school you would be able to do specific missions concerning that school and learn spells only that school can learn.
This is a very good question!!! I think we have already gone over this many times in the W101 central forums. If not that is an intresting question that ill look into!
This gets brought up a lot, and it's been posted here and on fansites for a long time. Since KingsIsle hasn't put seven wizards in, it's probably not going to happen.