And yes, I also want to know what turns a 500MB photo into >16GB in memory. That's 32 full-res copies of the photo in memory. Just "a couple layers and a couple undo steps", really?
Layer size: Layers add more data on top of this: alpha channel (8-16 bits/pixel), clipping mask and maybe half a dozen other things, easily bumping the whole thing from 6 to 9 or 10 bytes per pixel.
Layer count: It's fairly common to have multiple layers that are copies of the original photo, with different processing applied and blending them into the final one. I'm very much an amateur and when I'm serious about just making a photo look (not even creating something new) I end up with around 10 layers.
Undo step memory: a lot of work in the photo processing workflow is global: color correction, brightness, contrast, layer blending settings and modes, filters (including sharpening or blur) apply to every pixel of a layer. Each confirmed change (by releasing mouse button / lifting stylus / confirming dialog) is likely to have to store an undo step for entire layer.
Of course you can just persist some of that to disk, but if a single layer/undo step can be 800MB this will hurt productivity - only very recently we got drives that can write this much fast enough and that's why just a couple years ago, when having enough RAM was not really an option a lot of pro photographers had 10 or 15k RPM HDDs running in RAID0 in their workstations.
For the layers/undo steps, the "couple" of each was not my phrase. It was yours. If by "a couple" you actually meant dozens, then sure, maybe an 80 mpix photo really needs that much memory to process.
Presumably that's 16-bits for each channel of Red, Green, Blue. Since every pixel is composed of the three RGB components, that's how you get 48-bits for every pixel (and internally, Photoshop refers to 16-bits per color channel as 'RGB48Mode').
Additionally, as for your original question: For hobby or even 'normal' professional photography, I'd guess none. But rigs like the ones used for Gmaps Street View, the recent CNN gigapixel, etc. would probably have the capability to approach that size.