I was adding Large Text Fields within the contact field area and has been blocked of further adding after 5 Large Text Fields. After checking with Oracle, I received the information that this is "by design" and can´t be changed. Further, we found this article, where the limit is explained:
It would be great to have more Large Text Fields available within the Enterprise Licence to support the client the best way. We have a lot of information to store from the webshop and here Large Text Fields are often a must have.
It´s not possible to work with Custom Objects here because we have all contact related fields in the contact area. This has been created in this way to have a better performance for the daily imports of the updated contact information from the webshop and not blocking the whole instance the whole day.
Looking forward to have an enlargement of the limit in any future release.