This applies to the Scarlett 18i20 (All Generations)

The Scarlett 18i20 may only have 8 analogue channels built into the unit, but you can actually record up to 18 channels simultaneously on your computer. This is as the unit also has two types of digital connections; S/PDIF and ADAT.

The S/PDIF connection can send two channels of digital audio into the 18i20 via the RCA coaxial connector on the back of the unit (Optical S/PDIF available on 3rd Generation). There are many different types of devices that have S/PDIF outputs that you can connect into the 18i20, these include; 2 channels mic-pres and analogue converters like the Focusrite ISA One with its digital card.

The optical ADAT connections on the back of the Scarlett 18i20 can be used to send or receive an additional 8 channels of digital audio. Examples of devices you could connect to the 18i20 over ADAT are the Scarlett OctoPre and the Clarett+ OctoPre Please note that only at a sample rate of 44.1kHz and 48kHz can you send all 8 channels, at 88.2kHz and 96kHz you can only send 4 channels over the one connection (the Scarlett 18i20 3rd Generation features Dual-ADAT ports which allow you to get 8 channels at 88.2kHz and 96kHz). 

More information on setting up an external ADAT preamp can be found here. 

When connecting multiple digital devices together you need to always make sure they are synchronised and locked together. To do this with a Scarlett 18i20 you need to set the Sync Source in Focusrite Control (Scarlett MixControl for 1st generation) to be the digital input you are using and make sure both devices are at the same sample rate.

When using these additional digital inputs on the Scarlett 18i20 they will appear in the following order in your DAW;

Scarlett 18i20 (1st and 2nd Generation)

Input 1-8: Analogue inputs on the unit,

Input 9-10: S/PDIF inputs,

Input 11-18: ADAT inputs.

Scarlett 18i20 (3rd Generation)

Input 1-8: Analogue inputs on the unit,

Input 9-10: Loopback inputs,

Input 11-12: S/PDIF inputs, 

Input 13-20: ADAT inputs.