Many games available on GOG were originally distributed on multiple disks. Is there any easy way to reliably determine how many disks the game was originally released on?

One requirement: The method must not require visiting any site that we would not be allowed to link to here. (In particular, this means no linking to sites that provide the disk images of each disk for download, as they are illegal in most cases, and linking to such sites is against the rules here.)

https://www.mocagh.org/ can be a good resource, or even mobygames if you need that info.

When DVD's became standard? Around 2005? For older games take download size (in MB) and divide by 700.

I guess it depends on whether we are talking about single sided or double side, single density or double density disks, and what capacity the disks were designed or formatted to hold...

I remember UT2004 coming on 3 or 4 CDs. Half-Life 2 came on 5 or so. Funny how DVDs became standard on consoles before PCs, but I guess computer games being released on CDs was for legacy hardware or people without a DVD drive (we actually had one of those, it was a separate tray).

That's only going to work for games that are new enough to have been distributed on CDs. Many of the games I'm interested in checking were released on floppy disks, which are a lot smaller than 700 MB.

That's probably going to depend on how it was released. Some games came packaged with two different formats in the same box. Archive.org has a collection called goodolddays.net which provides concrete information. I won't directly link them, but they're there to peruse if you're willing to look them up.

https://ogdb.eu/

For example, the main entry for 1992's Das Schwarze Auge: Die Schicksalsklinge (Realms of Arkania: Blade of Destiny):
https://ogdb.eu/index.php?section=title&titleid=2693

The release versions for each region (in this case: Germany, Germany/Austria/Switzerland, USA, Worldwide) are listed a little further down, and on the page for the German 3.5" PC MS-DOS version it says under "Weitere Informationen/Inhaltsbeschreibung" (further information/content description):

5x 3,5" Diskette

As you can probably tell from the site being in German and the .eu domain, the main focus is on the European market and therefore the "further information/content description" section of the US release versions, for instance, often may be lacking the number of disks the title in question had been released on.

I've never found a website/database (like mobygames e.g.) that keeps track of all aspects of all PC game physical releases.

Phantasmagoria was on 7 CDs.

That information is not going to be consistent because some games were released on multiple types of media. What would the purpose of the info be anyway?

What is the purpose of that question?

To understand if there is any useful application for that data.

Take installed game size and divide by 1.44 meg? Not accounting for zipping up files or Galaxy fluff.
I don't know about hard drive file systems like NTFS today.

Wolf3d, Doom, Heretic would install and then de/inflate(unzip) their files. I can't remember exactly how many floppies they would take.
Wolf3d- 4/5? All 6 episodes.
Doom- 3? Originally came with 3 episodes. no 4th episode. It would be available as free patch.
Heretic - 3? Originally came with 3 episodes. no Shadow of the Serpent Riders episodes. It would be available as free patch.
Duke Nukem- Likely came with 3 episode disks.
Monkey Island- 7-12? All floppies, no hard drive. There's even a easter egg about inserting disks 17 or 36?, floppy disk version only. I panicked one time, silly boy.

While storage amounts for media are good to know, overhead of filesystem is required too... As i doubt you're using dd to direct copy a whole media's storage amount. As such floppies are almost always formatted with FAT12.

I know with resizing disks and changing how big the formatting is sector-wise you can push 1.44Mb to 2Mb or higher 'superdisks' but i never saw any in distribution nor could i get my own to do that. Which is too bad.

For 1.44Mb floppies after the Fat12 was put on, it is closer to 1.37Mb, 700Mb CD's become closer to 650Mb, and 4.3Gb DVD's are closer to 3.8Gb. Optical media also tends to include error detection codes, though how useful they are is hard to say.

Also older games you probably exclusively did zlib type compression, or zip related. Meaning the compression of current games can skew how much space the original sizes were by quite a bit vs 7zip that's used right now. And since zips tend to compress per-file, similar blocks of data won't get nearly the compression. (And i've tried no compression on a bunch of HTML's to a zip, then zip'd that, and got better compression than just zipping the original file with higher compression options) So probably expect half again more disks, unless the data wasn't compressible very well then it won't matter. True. But then you're getting older than 90's probably.

https://en.wikipedia.org/wiki/Floppy_disk#Sizes,_performance_and_capacity