It happens that people already know for sure, then it’s good, and you don’t have to read any more.
If it is not clear, then I would recommend buying 1 3.5" 4TB hard drive. This size is usually enough for all personal documents, photos, and even a small video archive.
You need to buy a second hard drive, but it’s better to put it not in a mirror on the same computer, but separately. For example, an external hard drive (plugged in from time to time), in a NAS in another apartment, or in a public cloud with encryption (here it is no longer buying a disk, but renting it). This approach will provide greater data security than 2 hard drives in one server, and downtime at home is not so important.
Thus, it is better to take a NAS for 2 disks - they are usually small and quite cheap, the second disk can be purchased if there is a shortage of space over time. There will be more problems with a single-disk NAS - and the disk is more difficult to change, and it will not be possible to expand by adding, and it will not be possible to make a mirror if necessary. NAS with 4 or more disks usually already takes up a lot of space and costs significantly more. Usually, such a volume does not matter for the home (except for the home video archive).