What is the difference between the Scarlett 18i6 and the 18i8?

Unlike the 18i6 the Scarlett 18i8 has four mic/ line combo inputs to let you record up to four microphones without the need of any extra mic pre-amps.

It also contains another two outputs in the form of a second stereo headphone output. The two pairs of headphone outputs will appear in Scarlett MixControl as outputs 3 & 4 for headphone output 1 and 5 & 6 for headphone output 2.