If I was building it from my 5 minutes of googling, using 15TB nvme u2 drives, and easily available server chasis, I can get 24 drives per 2u of a rack. That's 360 TB + a couple server nodes. So ~6u per PB. A full height rack is 42u, so 6-7PB per rack once you take up some of the space with networking, etc. So dozens is doable in a short datacenter row.
Realistically you could fit a lot more storage per U, depending on how much compute you need per unit of data. The example above assumes all the disks are at the front of the server only, if you mount them internally also, you can fit a lot more. (see Backblaze's storage pods for how they did it with spinning disks).
Probably an order of magnitude or two more. Still something that is feasable in a research context - early MRI and genome sequencing had similar "too much data" problems like this, but the researchers still built it out to learn stuff. Tech marched forward and these days no one really blinks about it. I presume that if such a "all the cells scanner" was invented today, it would only be used for research for a long time - and that by the time it became widespread data storage will have caught up.
If I was building it from my 5 minutes of googling, using 15TB nvme u2 drives, and easily available server chasis, I can get 24 drives per 2u of a rack. That's 360 TB + a couple server nodes. So ~6u per PB. A full height rack is 42u, so 6-7PB per rack once you take up some of the space with networking, etc. So dozens is doable in a short datacenter row.
Realistically you could fit a lot more storage per U, depending on how much compute you need per unit of data. The example above assumes all the disks are at the front of the server only, if you mount them internally also, you can fit a lot more. (see Backblaze's storage pods for how they did it with spinning disks).
Dozens of PB is not that much data in 2023.