There is a limit to the number of custom fields you can add. The maximum number can depend on the location (lexical entry, lexical sense, Notebook, and so on) and writing system(s) you select when you add the custom fields.
Tests show that you can add at least 100 custom fields.
However, the need for a large number of custom fields should prompt you to reconsider what you are trying to do. A large number of custom fields is not normally needed. If necessary, request advice from a language consultant, an experienced peer, or get technical support.