The LCNA exam is 34 questions.
The LCNS exam is almost 300 questions. They are all multiple choice, and can be questions on any supported operating system.
You can expect the following: performance questions, questions related to SAN, DDS, cluster, firewall, network (eg. router tables), and NDMP. The whole point of an LCNS certification is to ensure that you can use NetWorker without being supervised or needing help.
There can be questions where you will see a NetWorker configuraion, and you will be asked to determine why backups may be slow, or how you can improve performance.
If you have the Legato 640 Advance NetWorker course, study from there... that should have more than 50% of the questions covered. The rest will come from the Administrator guide.