OK, well you still only really have 4 inputs unless you have a SPDIF mixer or pre amp on hand. And you were after 6 or 8, so you might have to upgrade to the 18i8.
Like you were originally saying the advertising can be misleading with some brands.
Like the 18i8, unless you use the digital and Adat connections you only have 8 analog inputs. So 18 inputs is a bit misleading for most people.
And for some folks if you don't have a dedicated pre amp or a little mixer, the line inputs often found on the back, are useless for microphones.
Example- Your 6 input 8i6 is only good for 2 mikes without using additional equipment.
Buy the time you pay for the additional equipment you are often better off buying the right interface with the exact connections you need. The more wires and connections, the more noise in your signal path.
Many seem to make this mistake when purchasing an interface.
I have the Scarlett 6i6 which is more or less just like yours. I can use all 6 inputs together. I have a Yamaha 01V digital mixer so I use the SPDIF inputs primarily for my collection of hardware synths and some mikes. I have dozens of things plugged into the 01v so I never have to re patch most instruments. I also use it for the headphone mix.
I have a TC BH 250 Bass pre amp into one line in #3 and A Joe Meek threeQ pre amp into line in #4 that is my main vocal input now.
I actually don't use the front panel XLR inputs unless I have more than on person in the studio.
So I leave those open as they are nice and fast to set up with just about anything Mike or DI.
I never liked them for vocals.