This is how much I could pack on a standard (CR80 ID) card sized plate, while still being able to punch the holes without power (or precision) tools. (The aspect of ease of creation with household items was paramount for my design.)
There is also the problem of long term durability. Not being a construction engineer, or metallurgist, I have no idea what is considered "survivable" in case of an unlikely accident (such as fire, flood, crash, etc.), especially if one uses aluminium as the material. The more holes, the smaller the holes, I think the less likely chance they wouldn't degrade easily if not treated with care. (Aluminium is cheap, easy to work with just a cutter, screwdriver, and ruler.)
Moreover, 128 bits is plenty enough for encryption purposes. The AES standard uses key sizes of 128, 192 and 256 bits, but at the moment even the 128 bit keys provide plenty enough security.
Thus, if one needs to store more than 128 bits, one could just use a standard encryption tool like Age or GnuPG, and backup just the password. (The encrypted file could be mirrored all over the place without fear of brute-forcing.)
I had to look it up, an IBM card format 80x12 is 187x83 mm. Roughly 0.06 bits/mm^2. The CR80 is 86x54mm for the 128bits, 0.03 bits/mm^2. Not a terrible difference for accommodating hand tools.
Edit: If my first search results are to be believed, the first magnetic hard drive had a density of 3.1 bits/mm^2
There is also the problem of long term durability. Not being a construction engineer, or metallurgist, I have no idea what is considered "survivable" in case of an unlikely accident (such as fire, flood, crash, etc.), especially if one uses aluminium as the material. The more holes, the smaller the holes, I think the less likely chance they wouldn't degrade easily if not treated with care. (Aluminium is cheap, easy to work with just a cutter, screwdriver, and ruler.)
Moreover, 128 bits is plenty enough for encryption purposes. The AES standard uses key sizes of 128, 192 and 256 bits, but at the moment even the 128 bit keys provide plenty enough security.
Thus, if one needs to store more than 128 bits, one could just use a standard encryption tool like Age or GnuPG, and backup just the password. (The encrypted file could be mirrored all over the place without fear of brute-forcing.)