FreeNAS, Do I need 1GB per TB of usable storage, or 1GB of memory per TB of physical disc?

With ZFS, it’s 1 GB per TB of actual disk (since you lose some to parity). See this post about how ZFS works for details. For example, if you have 16 TB in physical disks, you need 16 GB of RAM.

Depending on usage requirements, you need 8 GB minimum for ZFS. UFS requires much less.