Alright, did some research.
Discworld was never released on the Amiga, and the PC floppy disk version didn't have the speech of the CD-ROM version, so the "16 disks" for Discworld is probably a warez release, so it's discounted. (The actual floppy disk version of Discworld was 4 floppies IIRC.)
(Of note though is that there was a "Director's Cut" edition of the original Discworld, which fixed some bugs and added an extra scene, but this version was apparentely never released.)
Willy Beamish and Bloodnet were indeed released on 12 floppies for the Amiga, but so was the regular version of "Bling! Sex, Intrigen and Skalpellen". They're all outdone by Rise of the Robots (13 disks) and, as
made correctly stated, by the AGA version of the aforementioned "Bling!" which shipped on 19 floppies. I couldn't find any other Amiga titles that shipped on more disks than that.
Meanwhile, over in OS and programming land, Windows XP was shipped to at least one user on floppies - over 250 of them! MS Office 97 came on 55 floppies, OS/2 Warp 3 came on 25 (39 if you counted the bonus pack), Slackware Linux came on 33 floppies and Lotus SmartSuite on 23.
EDIT: Typo correction.