The Scarlett 18i20 may only have 8 analogue channels built into the unit, but you can actually record up to 18 channels simultaneously into 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. 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 it's digital card.
The optical ADAT connection on the back of the unit can be used to send an additional 8 channels of digital audio to the Scarlett 18i20. Examples of devices you could connect into the 18i20 over ADAT are the Octopre Mkii and the Octopre Dynamic Mkii. 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.
When connecting multiple digital devices together you need to always make sure they are syncronised and locked together. To do this with a Scarlett 18i20 you need to set the Sync Source in Scarlett MixControl 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;
Input 1-8: Analogue inputs on the unit,
Input 9-10: S/PDIF inputs,
Input 11-18: ADAT inputs.