First question : budget ?
Second question : distance to the band ? (meters)

Suggestion without more details would be :
HDSPe Madi card plus a 12Mic

I ask for the distance because adat is shorter than madi.
Instead of a 12Mic you can take 2 OctamicXTC gives you 16 inputs.
But also the 12Mic can get expansion with an adat unit easy.
Madi solution gives you freedom to expand and modify the set for every situation.
Clubgig or big festival, no problem, and with an RME interface you have Totalmix the perfect tool for liveshow mix.
No need for any DAW.

I think you should also get a recommendation from companies that have some experience in the live sector.

This forum can support you here with regard to
- RME device selection
- telling you about the very good/unique RME specific features that are so extremely useful
You can be sure that RME devices and TotalMix are a very solid foundation for such a setup.

But the entire setup / concept / design of such a solution should better be in place beforehand.
This is maybe somewhat beyond the scope of a forum.

RME Features / Manuals

There are many possibilities to set this up, here some suggestions and introduction of features, where you should have a look at and inform yourself by asking here further or reading the manual sections
UFX+: https://www.rme-audio.de/downloads/fface_ufxplus_e.pdf // https://www.rme-audio.de/downloads/octamicxtc_d.pdf
Octamic XTC: https://www.rme-audio.de/downloads/octamicxtc_e.pdf // https://www.rme-audio.de/downloads/octamicxtc_d.pdf
12Mic: https://docs.rme-audio.com/shared/_atta … online.pdf

ADAT vs MADI

ADAT can usually go up to 16m although the standard says ~10m. But I also would prefer a MADI based solution where you can build up a ring of devices where each of the cables can be up to 2km long.

A combination of UFX+ and 12Mic would give you 4+12= 16 Mic preamps.
But then you would also need 4 longer XLR cables which might be uncool, depending where PC/UFX+ is located.

UFX+ & two Octamic XTC via MADI

In this case it might be more beneficial to use UFX+ and two Octamic XTC for two reasons:
1. then you can distribute the two XTCs better on stage as needed (more center or left/right)
2. XTCs can be setup to work sample exact, so that you can even connect a stereo Mic to both devices
3. with the XTC you have two (in total 4) Mic outputs where you can give some people a monitor mix
4. you can remote control the two XTC via Auxdevice support (via MIDI over MADI) and you can store / recall all settings digitally.

DURec Feature

With the UFX+ and DURec you can additionally connect a harddisk or USB stick to make a backup recording.

AUTOSET Feature

UFX+ and XTC support AUTOSET which you can use to find the correct input gain on the inputs easiert. Later you can decide whether you prefer to keep on turned on or off. On the one hand it can adjust the gain down, if an unexpected peak happens. But if somebody only accidently bumps to the mic, then it can also ruin your recording, if the gain is not being ajusted back. So I would use it for finding proper levels and then turn it off.

MADI Delay Compensation

XTCs also supports delay compensation so that audio from all inputs arrives sample exact at the DAW. If you connect only 1-2 devices no big deal. But remember, in such a setup you can "chain" up to 8 XTC's (for 8x8=64 Mic @44.1/48). In larger setups a latency compensation can make sense.

Delay Compensation - AutoCA:

Also very useful bewtween UFX+ and several XTC. The XTC will automatically choose their MADI ID and the Channel assignment / routing (XTC <-> MADI) will be performed automatically according to the position of the device in the MADI chain.

12Mic

If you prefer the 12Mic then you need to wait, Auxdevice support is promised but not yet implemented. You could remote control the 12Mic then through an ethernet/LAN or even WLAN, but then you have to work with two tools, TM FX and another remote control tool and you are not able to store such a setup digitally in TM FX Snapshots/Workplaces.

For the connection of active monitors / full range monitors, which nowadays can achieve enormous volumes / sound pressures, I would also strongly recommend you to throttle the volume at the active monitors so that a certain maximum volume can never be exceeded, e.g. by using attenuators maybe in combination with an analog monitor controller, see here:
https://forum.rme-audio.de/viewtopic.php?id=25399

The question is, whether you use your own active monitors or if you reuse the monitors of the location.
If you have an analog mixer there you can use this to control the final volume.
The UFX+ gives you a lot of possibilities to make a connection through analog balanced or even unbalanced.
You can also use AES which allows for longer cables lengths.

If you plan to use TM FX as a live mixer, you should always keep in mind that you might accidentally set the output faders to 0dB either by double clicking to an output fader or recalling a wrong Snapshot. The consequence
- broken ears and / or speakers and
- additionally the danger of feedback (+)

(+) Therefore, you should mayvbe also think about the use of a feedback controller. I suspect about the 3-band EQ in TM FX could be too fiddly for live use from the setting and in a live use it is also not bad to have real buttons on which you can quickly turn intuitively, especially if the number of mics is already 20.

You also have to make sure that you get the appropriate cable, because OM3/OM4 multimode cable would then also have to be laid and protected accordingly, so that nothing breaks.