I need 8 more chanels at the moment.
The focusrite 18i20 can double as a sound card for a second computer.
I would think that I can do both jobs with this.

I have seen that too.

but
I like the idea to have something that can double as a sound card for a the second pc

me too.
But I don't care. Main i/o will be on the BBF.
I need the 18i20 (if I get this one) for extension and/or for a second computer

Yep!

I was about to write that you(@d.panagiotidis) should, better, stick with RME and then I saw Ramses' post about the UC.
Get an RME interface that can be used in Stand-alone mode. This way, you won't miss the excellent drivers and the AD/DA will remain top notch!

Another recommendation is that you can buy another BF Pro FS and the Behringer ADA8200, so that you will, only, need to exchange the Behringer between the two!

Do you mean getting the Behringer ADA8200 and a DF Usb?
Absolutelly! Yes!

Another possibility:

Computer A: BBF (clock master)
                      +  +
                       |  |   ADAT / TOSLINK (IN+OUT)
                       |  |
                      +  +
Computer B: UC (clock slave)

Then you can e.g. route analog ports of UC through ADAT to BBF and vice versa.
Galvanic isolation through ADAT optical.
TOSLINK cable may be 10m long according to standard, but with RME I experienced so far that even 15+1m are possible between two RME devices. Use TOSLINK cable of Mutec, they have a very good price/performance ratio and are not to thick/stiff.

In my last example the 2nd machine could be used no matter whether the PC B would be on or off.

Szenario 1 - PC B off
- UC would operate in stand-alone mode and route its analog ports (as you need) to the BBF

Szenario 2 - PC B on
- UC could be administrated by PC B (driver settings, TM FX)
- I assume UC on PC B could be remote administrated by TotalMix Remote on PC A (*) or
   by any remote management software through network
  (*) TotalMix FX and TotalMix Remote can not be used to control the same interface on the same machine

Szenario 3 - disconnect UC from PC B and use it in stand-alone mode close to the BBF.
Just in case the distance is too high (as TOSLINK is limited to max 15m).
But as there are also 20m TOSLINK cables being offerned, you could try also 20m, maybe it still works for single speed.
https://www.thomann.de/de/mutec_optisches_kabel_20m.htm

> Since I have never hooked two adat machines can you tell me why we need both in/out optical of BBF?

Data transfer in such a TOSLINK cable is unidirectional.

Both devices need to be clock synchronized to avoid crackling audio / audio loss.
The BBF does not support WC (word clock) so we need to send clock from one to the other device through ADAT.

Assumed the BBF is / stays your main interface, it would also be the clock master in such a setup.
The clock master can send clock information only through an outbound digital port.

So the 1st TOSLINK cable we needm to send audio from analog ports (line, instr, mic) of the UC to the BBF.
The 2nd TOSLINK cable we need to send audio from BBF to UC (if this is applicable) but 1st of all to send clock information.

In the RME driver settings you need to set the following
- BBF: clock source: "internal" (so this is the master)
- UC: clock source: "ADAT or optical" (this is the clock slave who gets clocking info through ADAT IN)

In addition to my recommendation I would personally prefer the combination of BBF and UC (not MADIface USB),
because BBF and UC use the same ASIO driver, the Madiface USB uses the newer MADIface USB driver.

As you might know, a DAW can load only one ASIO driver.
So if you would get the BBF and the UC and connect it both to one PC, then Cubase could access both recording interfaces through this one driver.
This would have the major advantage, that you could even
- use all ports of all interfaces (ADAT supports only 8 ch)
- use higher sample rates without loosing ports (with ADAT the amount of channels halfens with double speed (88.2/96)
In such a setup you need
- to connect at least 1 TOSLINK cable between these devices for synchronization
- ensure in the driver settings to use the same ASIO buffer size ...

Because this offers you more options and even access to all analog ports with single, double and quad speed
I would favour the solution with BBF and UC instead of combining BBF with Digiface USB (my personal view).

Well it depends of course also on costs ... So if you have a real fix budget limit that may not be exceeded then please communicate that, then it makes no sense to discuss more expensive solutions even if they are smarter.

> Another reason is that I do not like what I recently read about rotary encoders and their problems

I wouldn't generalize this. I had already many devices without any encoder problems: UFX, UFX+, XTC, 12Mic, ADI-2 Pro, ADI-2 DAC, ADI-2 Pro FS R BE.

> Also their price does not go down

Sure of course. Look at the used market prices of other even older interfaces: FF400, FF800, Babyface.
Driver and build quality, long term software support, everything is being flashable in FPGA.
No lame or error prone 3rd party communication chips, all being done in the FPGA.

> Spending 200 for the behringer is cheap

Yes, but then you have still no recording interface for the 2nd PC.

> 400 for the focusrite manageable

I hope you do not mean the firewire version .. Firewire is a "dead horse".

The  Focusrite Scarlett 18i20 3rd Gen costs €459. OK, this is an USB recording interface but, spending so much money for a recording interface with more compromises than advantages and with a lame mixing software, etc.
I personally regard this as a waste of money.

Maybe the best option for you would be to get the ADA8200 1st as extension IF you need more than 4 Mic inputs in total
and then later maybe a used babyface for the 2nd PC at a later point of time.

Otherwise I would still prefer the combination with a UC .. as the combination of both interfaces offers you best quality and flexibility as both are RME recording interfaces with TotalMix FX, TotalMix Remote, DIGICheck, ...

I would only get the ADA8200 if you really need more than 4 Mic inputs.