Just wanted to add some info that might be considered useful here:
First off, there's a tool on a popular NAS vendor called Synology's website that I like to point people to when they're trying to conceptualize RAID configurations:
The Synology RAID Calculator. You just add in the quantities and capacities of drives you want, and select a RAID type from the dropdown to see how the storage space works out. Ignore the SHR options unless you have a Synology NAS because they're proprietary.
Secondly, it's important to think of RAID less as a backup and more as a way of preventing downtime. Lets say you run an online business, and you have a server which handles your transactions. A drive inside said server bums out. You can't afford to take the server down for hours/days on end because that's literally money walking out the door. RAID allows you to swap a drive/drives out and rebuild the data from the info saved on the other drives without the need to "take down" the server. It's about keeping the system functional. However, numerous things can happen while said data is being rebuilt with the new drive: New drive can suffer infancy failure (remember that historically, drive failure happens on a bathtub curve), another existing drive in the array can fail, amongst other possible scenarios.
For this reason, it's important to think of backing up important data with the industry-recommended 3-2-1 strategy: Keep 3 copies of your data, 2 locally but on different mediums, and 1 offsite in case of disaster. You can think of RAID as 1 copy of the data, but not as 2 or ever as all 3.