Not necessarily. A mixer really isn't needed at all if only recording an input(instrument) or two at a time. If you only had an interface that can record two tracks at once, then you would need to 'sub-mix' if there were more than two mics/inputs. The 1800 gives you enough inputs to record a whole drumkit at once. I do however run out of tracks myself and use a mixer to 'sub-mix' my guitar, bass, and vocal 'scratch tracks' into one track, saving room for more drum mics.
I hope that made sense.