Depends on use case.
For generic hoarding, most likely one 6-wide Z2 + 1 3-wide Z1 + 1 hotspare.
For anything with any performance component to it at all... well, you know me and mirrors. So either 5 2-wide mirrors, or 4 2-wide mirrors + 2 hotspares, depending on how nervous you are (and how well monitored you are).