Answered by Kristi Icaza

Ringtail will allow as many reviewers as you need to log in and review documents in your case database. For best results, use Review Setup to streamline document assignments to your reviewers.

If two reviewers open the same document at the same time, which isn't likely, the last change made will be the one that is saved.  You can prevent reviewers from "bumping into" each other by using workflows!

For steps, see Working with Workflows.