The Apollo 8P has 8 Unison mic preamps. Adding expansion via its two ADAT I/O ports on an 8P could get you to 16. In this case, you would need a second interface with A/D and D/A conversion because the 8P has only 8 channels of conversion. As stated above, in this setup case, you'd need
ADAT I/O. Finally, you'd need to decide which unit would be the master clock and set that up for both your Apollo and the ADAT expansion unit.
However, adding your 500 series preamps via line out on your chassis to line in on the 8P will replace the Apollo Unison mic pre with the 500 series mic pre. You either get to use one or the other, not both at the same time. So the most you could use simultaneously would be a combination of 8 mic preamps, not 16. Of course, you could use a patch bay to change which of your 8 preamps was active at any one time. But that would still limit you to 8.
Having said all of that, I agree with @
chrisso that the Cranborne 500ADAT checks all of the boxes you seem to require.
I don't own one so I can't personally attest to the quality I describe here. However, a lot of UAD Forum members use the 500ADAT and have described its quality this way.
It has the ADAT I/O you need and a whole lot more for $1,500 USD. You get an 8 channel / slot 500 series chassis,. You get 8 channels of conversion at up to 96kHz sample rates. So if you filled the chassis with mic preamp modules, you could get to 16 mic preamps available simultaneously.
You can use it as a master or slave with an Apollo using BNC Word Clock I/O. Or you could use the built in ADAT clocking if you want.
There are multiple analog routing capabilities including an insert send and receive on each slot. The insert slot capability could be a way for you to integrate your existing 500 chassis with a channel or two of the 8 slots on the 500ADAT. You'd use the analog I/O on your existing chassis to connect to the send and receive on a slot or two on the 500ADAT chassis's rear inserts. That would allow you to send a strip of modules with a specific order (say mic pre, EQ and compressor) to a 500ADAT channel. In effect, you'd have multiple modules routed to a single channel / slot.
On the 500 ADAT, you can put specific modules in consecutive slots and use switches to turn them into strips of two, three or more modules right from the front of the chassis. There is also an internal switch that can be used to bypass a slot until you have a module to fill the slot.
There are some really unique bonus features in the 500ADAT too.
500ADAT has an analog 8 channel summing mixer built into the chassis. That's a nice bonus.
And then there are 4 of Cranbourne's proprietary C.A.S.T. I/O ports on 500ADAT. C.A.S.T. is a CAT 5, 6, or 7 based networked audio solution that can be used as replacement for analog snakes. I can also allow you to add additional networked line level audio devices to the 500ADAT. You can read about C.A.S.T. on the Cranborne web site.
Note that the input sources can be selected from a front of chassis switch that selects your input routing options: ADAT (i.e. from your DAW), the current slot (analog) or C.A.S.T. In effect the 500ADAT can be used as an 8 channel routing hub that connects multiple 500 series modules or other analog gear to and from your DAW.
As for how do you use it? Depending on the modules you populate it with, it could be used for racking and mixing and mastering. Tracking use is obvious. But note that you could run a track from your DAW through any of the modules to add flavor simply by selecting ADAT as an input source. Or you could send your final mix through a stereo bus compressor for mastering.
