Depending upon they type of dive I am doing I will have several different "backups". I am redundant at several levels.
If I dive doubles, I have two separate regulator systems attached with an isolation valve. In addition to this I carry with at least a 19cuft pony rig (at least 30cuft if extended range).
If I dive singles, I have a regulator set (with octo), and a 19cuft pony.
Although I may appear to be totally self sufficient, I still dive with a buddy who is outfitted similarly. And my buddy had better stay with me.
Until I get my gill implants, I figure that I better take every precaution to make sure a lack of breathing gas does NOT become an issue while I’m underwater.