AES supports only two channels per standard, but this for sample frequencies from 44.1 up to 192 kHz.
AES does not allow for more channels.
You need to take ADAT to be able to transfer up to 8 channels @44.1/48 kHz.
When using ADAT please note that the amount of channels depends on the sample rate.
8 ch @44.1/48 kHz (single speed)
4 ch @88.2/96 kHz (double speed)
2 ch at quad speed, if quad speed is supported by the preamp.
Your Audient has two ADAT out ports, the UFX has two ADAT in ports.
If you record only with 44.1/48 kHz you only need one TOSLINK cable for the 8 Mic Preamps.
If you intend to record 88.2/96 kHz, then you need to use two TOSLINK cables between the two devices.
TOSLINK cables can be up to 15m long between certain devices, but I recommend you to stay with the standard
which is below 10m last time when I read a reply from Matthias Carstens ...
I think 10m up to 96 kHz should be safe.
When taking TOSLINK cable then I would consider to take not the stiffest cable ..
I made good experience with Mutec optical cable, you can even twist it a little so that it doesn't spread too much around.
Example article 5m: https://www.thomann.de/de/mutec_optisches_kabel_5m.htm
X10SRi-F, E5-1650v4, Win10Pro20H2, Cub11Pro, UFX+ (v0.9735), XTC, 12Mic, ADI-2 Pro FS R BE