You do not have to create separate accounts at www.syncloud.it they are ane account per device/dns name.
On a device you can create as many users as your resourcess allow using Users app. Can you post a picture of the error? Also ado you have any uodate available on Users app page?
All device share the disk storage by default. Apps can do their own restrictions, for example Nextcloud supports disk quotas if I am not wrong.