docs/system-admin-guide/custom-fields/custom-fields-faq/README.md
A hard limit does not exist. Nevertheless, there are factors that can represent a restriction in usability:
Yes, this is possible. When you edit existing work packages for which the custom field is activated but not populated you will receive the warning "[name of custom field] can't be blank" and you will have to populate the custom field.
You can find them when navigating to Administration -> Enumerations and clicking on an existing document category (or creating a new one).
Yes, you can. Select the custom field and use the Add projects button to add it to multiple projects at the same time.