Filters are sold and replaced in sets of two.

Our standard filter sets are MERV 7/8 for the warm side of the unit and a smoke/pollen filter, MERV 13 for the unit's fresh air intake side AKA 'cold side.'
It is suggested to check your filters quarterly with the seasons, vacuum 1x and/or change them at least twice a year. 

Each unit uses two filters at a time. Be sure to check the exterior grills for blockages at least 1x per year.