Took a trip down to the basement after making the previous post. Amazing what you can find it the “outdated box of goodies” and the memories they bring back (some good, some not so good).
Long ago I had a Thunder GC-HE S4520 setup as a server. Board was quad 603 socket based; I only ever had two in it. This thing has eight PCI slots (7 @ 64bit, 1 @ 32bit). Integrated Adaptec Ultra 160 SCSI controller as well as an integrated ATI Rage video chip (and dual LAN controllers). This was the foundation to a smoking machine at one point.
I also had another system based around a Tyan Tiger S1832 dual Slot 1 board. It only has five PCI slots but also includes, hold your breath, two legacy ISA slots! Also included, at the time, “blistering fast” AGP slot. Lots of dust on this one.
Found some notes on a SuperMicro P6DGH. Never had one but if the comments are correct, it offers nine PCI slots plus two ISA and one AGP. The notes don’t reveal why I never ended up getting this board. Might be worth trying to source via eBay or elsewhere.