I believe you're in the wrong forum, this is just for users of SQL Developer, for SQL Developer specific questions.
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(1) As Jeff already said your question is OT.
(2) My guess is that DataTable instance dt returns only a single instance e.g. a single data source.
May be the following note from .NET online docs applies:
Due to the nature of the mechanism used by SqlDataSourceEnumerator to locate data sources on a network, the method will not always return a complete list of the available servers, and the list might not be the same on every call. If you plan to use this function to let users select a server from a list, make sure that you always also supply an option to type in a name that is not in the list, in case the server enumeration does not return all the available servers. In addition, this method may take a significant amount of time to execute, so be careful about calling it when performance is critical.
I have 2 Oracle Server 12g on my local network.
...the database instances will have servicenames associated with them. Servicenames known in advance.
Use tnsnames.ora, have as many servers and servicename aliases as you like pointing at what ever you want to expose to "end user".
Define your connections to your instances/pdb's/cdb's on your different database servers in there, with meaningful aliases, and use that.
No point polling a Oracle db server to try and see what instances/pdb's/cdb's may or may not be accessible. Waste of time.
The DBA or who ever created the db servers and created the databases, etc... will already know the list.