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In regards to your question, I saw on a thread on Apple Discussions, that the limit is 50. However, that was back in 2008, and things may have changed. I'll keep sleuthing and if I find updated information, I'll post here on this thread!
You can enable Emergency Bypass per contact if you wish to do so.
Simply go to the contact you want to bypass Do Not Disturb > Ringtone and enable Emergency Bypass.
No need to add them to your favorites.