Its a 3y old thread and he was talking about different ideas how to connect his devices which all was valid.

So it would be good if you would describe YOUR equipment and requirements.

You can of course stack multiple cards  (as described in the respective handbooks).

The disadvantage of such a solution is that you loose the ability of any to any routing in Totalmix FX.
If you take ie three cards then you have to work with one TM FX instance per card (3 in total) and you can only route channels inside of each instance but not across TM FX instances.
This is only a good approach if you know that any to any routing is not required or if you intend to route inside of the DAW.
If you intebd to route route inside of the DAW then you have always the Round Trip Latency in the way.

A better approach could be to choose a recording interface which has all required channels.

There I would pick PCIe based solutions to not be limited in the selection of mainboards, because mainboatds withPCI you find only rare these days.

ADAT limits you to approx 15m cable length and with higher sample rates you loose 50-75% of channels by multiplexing.

So it would be extremely helpful if you would describe your setup, how many channels you intend to transfer and what sample rates you need at maximum. A product link to your equipment would also be nice.

Much more flexibility you have with MADI.
64 channels and still 32 when recording @88.2 and 96 kHz.

You can connect multiple devices to one MADI BUS, the cabling goes from one device to the other and back to the origin.
I recommend OM4 cabling which is also good for computer networks, then you can stick to one type of cable infrastructure.
Cables between each device may be up to 2km or even longer if you order the RME devices with Single Mode interfaces. As this is more intensive laser light with different parameters you need different type of cables and all devices need to have compliant devices.
More flexible you stay with the normal multimedia fiber interfaces.

MADI is also nice in terms of remote controlling devices (MIDI over MADI). Can save a lot of MIDI cabling.

RME offers different solution how to implement MADI: USB2 / USB3 / thunderbolt / PCIe based.

The top interfaces are IMHO
- UFX+ (USB3/TB) with 1 MADI bus offering also other types of ports and the very useful DUREC
- HDSPE MADI FX (PCIe) with 3 MADI busses, full implementation of FX chip, can act as MADI router, ASIO driver with resource optimizations and redundancy features

But there are also other interesting available.

Now it would be nice to know about your setup and requirements otherwise it's impossible to make a good recommendation.

Sounds like an easy question, but there are a few things to be considered.

What is BTW your budget ??