I know I've installed Win95 off of floppies. Don't recall the exact number, but believe it was around 40-50 3.5" floppies. It came in a box about the length of a shoe box.
OS/2 Warp was 40 or so floppies, and sold at such a discount that buying a copy was the cheapest way I knew to get high quality floppies when I was in college
I have somehow never gotten around to throwing away the box of ancient floppies I've got in a closet from ages ago, and the Windows 95b (OSR2) installation disks I made were still in it, complete with custom color printed labels I splurged on.
The media I copied from took up 28x 3.5" HD floppy disks. It's possible they were copied from what was originally a CD-ROM. I don't remember clearly anymore.
Note: I'm not trying to refute or correct your 13-disk figure, which was clearly a different installation set, and likely original Windows 95 rather than my OSR2, which came out around 1997.
Hrmm. Seems you're right and my memory is way fuzzier than I thought. Raymond Chen also says 13 [0]. Maybe it was just my existential dread of one of the floppies having gone bad that makes me think it was more.