You can create as many Wizards as you like on an account, however you can only have 6 at a time. If you have 6 Wizards now, you must delete one to create a new one.
Remember when you delete a Wizard, you also delete everything in its backpack (including all its houses) and bank. Items in the Shared Bank are not deleted when a Wizard is deleted.
Alternately, if you don't want to delete any of your current Wizards, you can start a new account and create new Wizards on the new account.
you can create multiple accounts and have more than 6 wizards. Plenty of people here carry many accounts and tie them together on a family account to lessen the monthly financial burden