Hi, welcome to the RME user forum.

To get a concrete idea about what can be done or what your (price) expectation is:

1- What is your budget for recording interface and AD/DA converter?
2- When talking about 20 analog outputs then you surely need the same number of inputs, right?
3- What else do you need? Mic Inputs? MIDI? Instr inputs?

Some comparisons of RTL of different products I put together here in my blog:
https://www.tonstudio-forum.de/blog/ent … cts-en-de/
I think it will be of interest for you to see, how close RME USB- and PCIe-based solutions are.

Cubase gives me latency compensation for each of the external devices that I am using (PCM18 and PCM91). It "pings" through the devices and then knows the exact latency through this device (hint: set it to 100% dry during such measuring, otherwise reverb and delay will lead to wrong/higher latencies). You should check for Protools and Logic whether they support the same for an easy setup as you want to use the external devices like a VST in your DAW if I understood you correctly.

I think we can assume that latency compensation of the DAW will take care of nearly everything.

But you should make some thoughts about how to set it up and use the external devices for each of your use cases.
Routing on the recording interface itself will happen in near-realtime with nowadays fast converters.
But if you sent audio one or more times over PCIe or USB, then the latencies might add-up, depending on how you organize your DAW project and the recording/monitoring.

EDIT1: two additions

If you want to use 500 lunch boxes, they are usually connected through analog ports.
The Cranbourne with ADAT are rather expensive.

Should you intend to record in double speed, then you loose 50% of channels.
If you were planning with a RayDAT card with (EDIT1:) 4x ADAT I/O then you would only have 16 channels at double speed.

If you think about 20 channels (or even more) and if you can't exclude that you might want to record in double speed now or in the future, then you should think about a MADI-based solution. This offers 64ch@single and 32ch@double speed.

You could perhaps think about getting an UFX III.
This gives you already a very useful mix of analog and digital channels (EDIT1:) where the number of analog channels stays regardless of chosen sample rate)
4x high quality / transparent preamps with 75 db gain which can also be used as Instr inputs.
8x high quality AD/DA converter.
2x MIDI I/O
Useful options like DURec (Direct USB recording) for either standalone or backup recordings).

And then you still have
1x AES (maybe for integrating an ADI-2 Pro FS R BE reference converter for monitoring)
2x ADAT (maybe for connecting one lunch box up to double speed)
1x MADI (optical or coaxial or mixed) for connecting AD/DA converter or preamps as you like.

The best alternative to that would be to get an HDSPe MADI FX.
This is simply the best PCIe card, offering 3x separate MADI buses (2x optical, 1x coaxial per daughter board).
You can get an optional daughter board to get a 3rd optical MADI port instead of coax.
It has 2x MIDI and one AES port which you can nicely integrate an ADI-2 Pro FS for monitoring.
The card has a phone output where you can connect phones or to use it as unbalanced outputs.

A quite expensive, but very well setup could be this.
Only to put an example what could be achieved.

PC----USB----ARC USB
|
| PCIe
|
HDSPe MADI FX
|  |  |  |
|  |  |  +---AES----ADI-2 Pro FS R BE------------Active Monitors
|  |  |                                              \------------Phones
|  |  |
|  |  +-MADI #1---M-32 AD Pro
|  +----MADI #2---M-32 DA Pro
+-------MADI #3---12Mic (via HDSPe OPTO-X card) (*)

(*) I recommend staying with MADI optical to have galvanic isolation between devices.

M-32 AD and DA could also be connected to one MADI bus because both devices use only 32ch (the one input, the other output channels). But as the HDSPe MADI FX has 3 (!) MADI buses, I would connect only 1 M-32 per MADI bus.
The forwarding of audio packets between MADI devices on one bus in a serial chain creates a latency of around 3 samples per device. This is not tragic. But as you have that many MADI buses with that card, you can distribute the two M-32 across two MADI buses and even avoid the 3 samples delay.

Normally, you can chain up to eight 8ch MADI interfaces in a serial fashion - one after the other - and for the old devices following an 8 port scheme (per device) RME invented MADI extension, delay compensation. This takes care that all packets arrive at the DAW with the same latency. Then you could, e.g., connect even stereo Mics to two different OctoPre XTC and audio arrived at the same time at the DAW, regardless of where you plugged it.

Now, with the newer devices with much higher port densities and faster AD/DA converters, delay compensation is not required anymore. But … With this amount of MADI buses, you can make use of all buses and organize it nicely this way.