"One" of something? You don't know me that well.
I usually never buy "one of something." It's usually in pairs my man -- I buy in pairs.
For us, backups are key in the location recording biz...
Well, it's really three of everything but, I hate odd numbers.
Let me explain. You start off with the main device, and then you clearly need a backup to that device and a backup to the backup.
So, let's round it off to four units since it rolls off the tongue much better than three.