There are various expansion chassis for Macs:

PCI macs to Nubus



https://68kmla.org/forums/index.php?/topic/13824-second-waves-pci-nubus-expansion-box-others/



Nubus to Nubus.    

Digidesign NuBus Expansion Chassis (12 Slot)


http://archive.digidesign.com/support/propix/DigiChassis.html

For PCI to PCI Magma is the leader:

PCI Expansion Systems provide an easy way to add multiple PCI and PCI-X cards and drive bays to your computer. A variety of expansion chassis are available ranging from 1 to 13 slot expansion with monitoring and redundant power options. Multiple systems can be daisy-chained for maximum expansion capabilities. Host connection types include standard PCI, Low Profile PCI, CardBus (PCMCIA), PCI Express® and ExpressCard™. Practically any type of PCI card can be used in the Magma expansion chassis ranging from audio processing, video capture, graphics, test & measurement, data-acquisition, SCSI, Fibre Channel, SATA and more.

http://www.magma.com/catalog/classic-pci-expansion

http://www.magma.com/13-slot-pci-expansion




Smaller 7 slot http://www.magma.com/7-slot-pci-expansion


I have seen also some SBS chassis, probably Digidesign/Avid rebranded.

It is important to make a picture catalog of them and the cards. Lots of PowerMacs are sold with one host card! On ebay I have seen lots of incomplete setups without cables or without PSU or only the motherboard on ebay by people who dont have a clue on what is that.
Please Knezzen post info about yours. From SonikArchitects (BT)

 I just bought a new 13 Channel PCI13-R chassis from Magma and was completely and totally unable to get my dual 1 to recognize this chassis. We'll Digi, Magma, no one could help. Instead of formatting as a story...I'll include my email to Magma today when I finally pwned this system.

...

I got it working. This is going to be a long email and I’d like to speak to you as I need to order/swap the cards and cables you sent me for older cards and cables. Contrary to the assertion that your devices do not have firmware/REV versions…they most certainly do and I’ve proved it by solving this problem.

SO:

The controller cards you sent to me (host and expansion cards) do not work with any generation of Protools 24 (Mix, Mix IO, Core, Farm etc) cards whatsoever, in any iteration. My Korg Oasis card is the only card that the chassis can see with the new controller cards you guys sent.

This card (I pulled from a 7slot Magma expansion chassis WORKS). However there is a very specific combination of placement, cards, order and specific versions of software that makes Digi Mix 24 cards recognized in a PC13-R Magma chassis.

Expansion Interface card
PCI-EXP 2.1
PYE 3255A4
01-03522-00 REV (left blank)

I need a matching pair of the above cards and 2 cables (one new and a spare) for my other system. I am happy to send back the cards I just purchased to swap for the older cards. I am also happy to pay for the cables.

I was told this system would work with my Protools Mix 24 system. It’s an absolute miracle I was able to figure this out and since I was very specific in my initial correspondence and made a purchase based on the assertion this would work, I am sure Magma will be gracious and accommodating.

Moving on, to make this work the controller card in a 5 slot G4 can ONLY be placed in Slot 2. It will not work in any other slot. After the card is seated properly you have to purge PRAM and reset the NVRAM. It will not work if you do not do this.

The (old) expansion controller card must be in slot one of the expansion chassis. The old (do not know how many pin) cable must be securely connected.

Next you MUST run the OS9 Magma card viewer application. I got this from your website. If you do not do this before running DigiTest the cards will not be recognized.

Finally you have to have followed the Digi recommended hierarchy of card placement. No other iteration that Core(s), Mix Farm(s), DSP farms (s) will work. I am still unsure how much bus power these will draw so until I’ve worked on the rig for a while, I will not know how many of these cards can be in the chassis and working stably.

Okay so next you have to run the very last version of DigiTest. It was made in August of 2002. It’s the last version of DigiTest in OS9. NO OTHER VERSION OF THIS APPLICATION WILL RECOGNIZE THE CARDS AND MAKE THEM AVAILABLE FOR USE. Once you run this, you test the cards…it all works.