12‽ I'd swear the Slackware I downloaded was closer to 30+. On dialup. Via a VAX. Using FTP to go from internet to the VAX box, then Kermit from the VAX to the DOS PC using Procomm Plus. Write it all, start the install sequence, find out that the 18th disk was bad. Reboot. Rinse. Repeat.

X disks were X11. There were also the A,B, C etc disks.

Then there was the Coherent install, with massive manual on ultra thin paper with the shell on the front.