Raid and Mirroring Redo logs-Newbie

I am relatively at Oracle. I have 2 questions.

1. We have an HP with raid 5. My Oracle instructor suggested that redo logs should be mirrored within Oracle. Is this really needed with raid?
2. The system will eventually have 2000 users. Most of which will have a login, but may not being actively working at the one time. Are there any rules to determine whether to run a multithreaded or 'normal' Oracle instance?

Thanks in advance

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