That's a thumb-rule. Depending on how you use the system you might need far more RAM than the thumb-rule, or far less. I've dealt with people that had 30TB of disk space and 256GB of RAM. I've also dealt with people that had 64GB of RAM and 100TB of disk space.
The thumb-rule is a guesstimate to start. If you are more experienced and you are building a system you will get a "feel" for what a good start is. This is where experience is helpful.
There is no cheat sheet that will tell you how much RAM you'll ultimately need. That's something the FreeNAS admin will need to figure out based on historical usage and experience. If you plan to build a system that is of that size, and you don't have this experience, you should consider paying someone as a consultant(or going to iX, or paying someone to build the entire system and sell you a pre-built setup) to answer these questions for you. Paying a consultant is often far far cheaper than trying to get it right yourself, especially with a server of this magnitude. I can't tell you how many $1000s of dollars of hardware people have bought that served no purpose.
This isn't Windows, and throwing more hardware at it like people do with Windows machines doesn't guarantee performance will increase or even stay the same(yes, I've seen performance decrease when people add $1000s of hardware).
