You can expand the number of analog ins and outs (up to the 18 and 20 in the product’s name) by using:
ADAT
Essentially, you would get a second box, typically a similar size to the 18i20. This connects over the light-pipe (fibre optic) connections.
This will give you an additional 8 in, 8 out [assuming you run at 44.1/48kHz sample rate - increasing rate = fewer channels useable].
Obvious products:
Focusrite Octopre
Behringer ADA8200
SPDIF
This will give you up to another 2 in/2 out.
Typically you might have little palm-sized boxes - one for D>A conversion (plugged into your ‘out’) to give 2 analog outs, one for A>D conversion (plugged into your ‘in’) to give 2 analog ins.
Obvious products:
Lindy
Swissonic (Thomann)
[search for ‘spdif a/d’ or ‘spdif d/a’, as needed].
I’d suggest going with ADAT extension in the first instance, then push to Spdif when you need it.